Just to answer part of your original question: Torflex is the name Dexter puts on its torsion spring type axle. Almost all the current horse type trailers use this today. I question that it is better than the old interactive two axle leaf spring set up (it 'walks' over bumps and holes very smoothly -- I don't think the Torflex does). I suspect that 'cool factor' and 'the market' have made them popular, though they also are extremely simple to align and install versus a leaf spring axle (4 bolts per axle versus welding leaf spring mounts on and aligning the springs).
Anyhow, we just sold a 25 year old Circle J two horse trailer. A lot of them out there for under ~$4000. Just like looking at a used car: If it looks like it was taken care of and there are no major repairs (frames, tongue, etc) they are probably good. Having said that, they will likely need a couple of these things: lights, or wiring, breakaway switch, battery for breakaway circuit, floor wood, etc. My wife bought it new and washed the interior out after hauling, and also put a stall mat over the wood (easier to clean, some protection for the wood -- which was not pressure treated). Still had great wood after all those years. I have gone over the wiring a couple of times, finally replacing all of it.
Feature wise, acknowledging the multi use stock trailer (note that a tall/draft model is so much better), a horse only model is really nice to have:
1. An escape door for a human (best to actually use a large 5/8~3/4 inch rope through the little door for the horses head; run it from the halter, through the door, and back to the human that is urging the horse into the trailer). This way if there is a 'wreck' the human can escape. Some trailers come with two, but you only need one -- you load the first horse in the stall without one, and the human can go into the other stall. Similar, the second load up the human can use the escape door.
2. We use trailers with mangers. This is the layout on standard straight load trailers. Not common on slant loads (but available). WE DO NOT feed while in the trailer. There may be a 'reward' of a handful of treats for loading (mindset that there is value in loading up-- food). There is always time to eat OUTSIDE where the horse can eat as it should -- neck out, head down (the way the intake tract is designed to be in eating mode). The manger gives the horse something solid to lean into while underway, as well as reserving a place for there head an neck to be.
3. Don't drive down the road with the head window open. Lots of sound, wind, and sometimes a horses head hanging out to be hit by an object (bug, bird, rock). On the road is not a place to need a vet. Not that there is any good place.
4. Not so big a thing, but we prefer chains for butt bars. Not so much to clank and move around if a horse is nervous, and you have more options while moving around to attach it.
5. The back doors should be of the 'dutch door' type. You close the bottom door upon the horse getting in (they can't back out then). THEN you reach over and attach the butt chain.
6. In a straight load, single horse stall trailer, we tether the halter with a breakaway tie. It has a quick release mechanism that is operated by sliding a collar away from the horse. In normal conditions, they can move there head only some, but not enough to be able to step back out of the trailer.
7. Having a tack compartment of some size (the old trailer was under the head/manger area) gives you a place to put two saddles, and enough tack for the horses, plus the emergency stuff, shipping boots (leggings: keeps the horses from stepping on/kicking themselves in the lower legs).
8. If you are going to be going somewhere overnight, you will want to be able to carry hay. On a two horse, that means hauling it on the roof (unless you are pulling with a pick up). Kind of a pain to load, but many trailers have the rack up there. Why not use the hay where you are going? Well, horses can be sensitive, so you want to bring your own hay, as having a horse with colic (you never want that) is a very serious thing.
9. You will want to be able to tie the horses on the outside. That way they can be saddled, etc. The area around where they are to be tied to needs to be safe, including on the trailer. Not usually a problem, but something to think about.
10. We camp a couple of times a year with the horses. My wife is AR (a good thing in a horse owner, as horses have a very tiny brain, and do one thing well: run; that's all you need to do when you are a herd animal with long legs and a small brain), and as a result of experiences and a former competitor in long distance trail riding (the organizations name is lot to me) she has several specifics for tieing them off and leaving them on the trailer: The lead should be tied so that the horses head can just barely reach the ground (it will stretch some when there is a 1000# pulling on it; this is to keep the horse from stepping over the lead and hooking a leg in it and having a wreck), the tire/fender area has to be filled (on the old trailer she put thick foam pads in there, including between the front/rear wheels; this keeps them from hooking a hoof and pulling a shoe, or getting a hoof caught under the edge); water is hung to the side of the head tie so that they can get a drink, but are not likely to spill it (usually the water tie is over the fender). An alternative to this is one I have seen (mostly) on stock type trailers: the fender is a long shelf that runs most of the way along the side.
11. Horses will want to be able to see each other when tied off. Let that help you position a tie off (lead to ground rule) so that they can see around the back corner. They stay calmer if they can see a stable mate.
12. Rear door latches: There are some that are more self latching than others. Mostly the best are only on fairly new trailers, but not necessarily on all the models of any given make. A bored horse will just fiddle around until the open it (it is amazing). Some trailers have ramps. Not a big fan, especially in areas that are cold (the ramp can be slippery). The horse will learn to step up/down, and in the long run will find this to be a more natural thing (plus, they can't step off of the side in a horse wreck).
13. A sign of a well thought out trailer is the hitch. I don't have any experience with goose neck or 5th wheel hitches. The best known of the good ones is Bulldog. It has a side opening latch assembly that wraps around the ball and latches closed with spring loaded collar. Strong, solid, positive latching. On bumper pull models you will find 2" or 2 5/16". Most hitches can be retrofitted.
14. If you have to change the wiring connector, upgrade both ends to the 7 pole blade. It has a 'self wiping' action that helps to keep corrosion cleaned off of the contact points. And it is the defacto standard. Makes it easy if you need to change pulling vehicles. That is also a reason to use a bumper pull trailer, you don't need a particular dedicated pull vehicle (suv, van, pick up, etc).
There are different opinions, and all have good reasons. I tried to point out the easier things to find and use. Especially as you learn what your likes and dislikes are. And your budget grows. We store our trailer such that it is accessible for emergency use. This means it ends up being at the house next to the driveway, so that snow and mud are not problems to getting the horse out (if the horse can't make it out to the trailer, it is too sick to trailer). I added back up lights to my trailer. It really makes it easier to move around at night. I used fog lights mounted so they point out to the side.
Oh -- and please have mirrors that are big enough to see around the trailer. You and everyone else will be much happier.
I'll add a couple of points, and disagree on a couple (-:
1) Ramps are OK, but (always a but) keep them closed up until you actually USE them if it is raining.
2) A horse will naturally try to look behind when backing out.
In a 2 horse trailer this has the potential to become a ramp issue.
The horse will TEND TO try and look over it's shoulder that is not against the wall.
UNFORTUNATELY if you allow this while backing out the hooves will tend toward the edge of the ramp.
So, when backing out lead the head along the trailer wall. The hooves will then tend toward the center of the ramp.
3) Butt bars (or chains) can be a hazard if a horse tries to back out by ducking it's butt under.
Try to minimize the exposure time by loading the horse that is most likely to stand calm and patient first and getting the ramp up (or closing the rear doors) as soon as both are loaded.
I don't ever leave a horse in a trailer with just a butt bar holding it in.
4) Yes, sort of agree on the torque-flex question except for the one BIG benefit which is that they only attach to the outside of the frame.
This means no "axles" running across the trailer, so a lower floor; resulting in lower loading height, shallower ramp angle, less step-up height, lower center of gravity, more stability, etc.
Whats to not luv ?
5) I have never had mangers, but have read horror stories of horses trying to climb into them. I use only straight load trailers and accept that a horse will destroy the fabric feed tray if it freaks out and tries to rush forwards or paw at it.
6) Escape doors; I open up both of them for loading.
It helps to create the illusion of openness and at least suggests that the trailer isn't a dead end trap.
On the head to head trailer I will sometimes open up doors and ramps on both sides and the rear to make the whole thing appear to be "walk through" for a reluctant loader.
7) Since I use ramps I always hook up butt bars before raising the ramp.
Never had it happen, but if a horse tried to back out while I am lifting a ramp things could get really ugly very quickly.
8) My PREFERENCE is the head to head trailer. Every horse gets to walk out forwards, any horse can be taken out without disturbing the others (at shows, then put back and others taken out) and the "face space" in the middle just has to be better than facing a corner.
9) Back up lights; I re-wire the rear loading light with a 2 way switch (center off).
One position is normal loading light, other position is to the back-up light wire.
It isn't THAT MUCH of a light to see with as much as it is a warning light to other drivers that I am coming backwards.
On the big gooseneck trailer I also added "tractor lights" on each side at the front and aimed back at the tires.
Rationale; Where those tires are about to go to is what I want to see in my mirror as I am backing up in the dark.
Mirrors: I respectfully disagree on the need for ever "seeing around" the trailer.
I cannot see vehicles narrower than 8ft if they are centered behind me, in fact with the gooseneck trailer they could be almost 8ft 6in and not be visible.
For BACKING; it is ALL done with mirrors and the one that matters is the one on the inside of the curve.
This is about the simplest description I have heard of how to back a trailer;
"Drive the trailer wheels that are on the inside of the curve."
They are the only ones you can see in your mirror anyway and if you get those around the curve right (assuming there is enough clear width) the rest of the trailer will go where it needs to.
It only takes a very slight bend between the truck and trailer to lose sight of the outside wheels anyway and the inside wheels are EASILY seen.
Not disagreeing for the sake of it, we just have different opinions on this.
