if ya down sizing, what are you getting away from? machines / implements?
will you ever go to larger round bails? are you building or have some sort of hay feeder for horses? that needs some sort of X amount of lift capacity either FEL (front end loader) or 3pt hitch to move it? or get hay into it?
what are you going to do with the manure from the stalls / lean to / run in shed? do you have any sort of manure spreader? or having a local farmer, using theirs, and spreading on a near by field? putting in a compost pile?
a general duty bucket for FEL, can work, but there are better "tine's / forked" buckets to deal with straw / manure that may build up within the run in shed for horses
what do you plan to do for the pasture / pad-dock areas? using rotatory cutter (bush hog), and then nocking down the manure piles / leveling things out?
are you needing to move pallets of feed, or use grinder for feed?
rock/gravel driveway? length of driveway?
how much area you plan to finish mow vs using a rotatory cutter (more folks call them a bush hog = brand name) finish mowing = general yard around house.
do you need to load/unload stuff from a trailer via "forks" aka stuff on pallets?
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more of above is trying to identify things that you might need, or finding min limits of what the tractor might need.
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doing your homework / research...
Tractor Attachments And Skid Steer Attachments For Any Tractor Or Skid Steer = has videos on all the attachments and a good amount of text to go with it. watch the videos and read through the attachments. to familiarize yourself of what is out there.
Used Tractors For Sale at TractorHouse.com: John Deere Tractors, used farm tractors and farm equipment, tractors for sale, Case IH, New Holland, Agco, Kubota = a lot of used stuff is sold there, but more stuff geared directly to farming / live stock / horses. look through the various things
TractorData.com - information on all makes and models of tractors = compare general specs of various tractors. it may not be perfect with tons of details, but a good start and way to compare the basics of tractors.
tractor, FEL (front end loader), cab, and backhoe, tends to be high dollar ticket items vs all the various FEL attachments or 3pt hitch attachments. if ya going to want them, it more likely better to get them all at once, hopefully dealer gives you a larger discount this way. and then save up as needed for the various other stuff as needed / wanted.
many of the add-on options for a tractor can be very useful. and in that having a larger tractor with extreme basics, vs a smaller tractor with all the options... can make a lot of the multi tasks easier, quicker, and get a more professional look, vs using a larger basic tractor. most of the bigger manufactures you can go onto there website and "build a tractor" with various options, go through them and learn about this or that. to help familiarize yourself with options.
for me... upper front and rear work lights (cabs normally come with these), a bigger alternator, more so if wanting more lights or wanting use an electrical winch on tractor, or run a small electrical power tool via 12v to 110v converter. TNT (top and tilt) for 3pt hitch (replacing top link and either 1 or both side links with hydraulic cylinders) 3rd function joystick for FEL, if you need a grappel or like for FEL, 2 rear remotes i would more likely min suggest, 3 if ya going for TNT for 3pt hitch.
the various tractor addon's can get rather expensive quickly, and if you are having dealer install them, it can be cheaper (hauling tractor back and forth, and labor) just to have it all done at once.
quick attach for FEL and 3pt hitch gets noted a lot on this forum... there is also telescopic lower lift arms for 3pt hitch, along with "pats easy hitch" to make hooking up to 3pt hitch stuff much easier. along with upside down U shape quick attach for 3pt hitch. quick attach may seem universal, but generally you will read folks slightly modifying a 3pt hitch implement to work correctly for a quick attach.
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i am guessing you will want to stay out of the SCUT (sub compact utility tractors), and go for a lower end CUT (compact utility tractor. there is a grayish line between SCUTs and going into CUT's. were ya get away from the overgrown riding lawn mower frame and look in a SCUT, and get a frame that sets higher off the ground (more ground clearance), and more over all weight. (getting into CUT's). with that guessing you will more likely want a CUT tractor to help deal with horses. granted a SCUT just for plowing, mowing, rotatory cutter could get the jobs done.