So would the Mahindra 3550 be a good choice? Comparing it to similar size TYM and LS, the Mahindra is a foot wider.
By now you seem to have found that there doesn't appear to be a a perfect "do-everything" tractor, the trick is to concentrate on the things that are important.
First, some history....In my life we've had a couple of rural homesteads with 5 to 40 acres to take care of and a variety of tractors through the years. And tractors themselves have changed. When I started out, most tractors had gear shift transmissions and only a few offered a power steering option - even with a loader. Canopies were folding canvas. Today a HST tranny (what a wonderful thing!) is common and most every machine has PS. The canopy is sturdy and often insulated. Seats are hugely improved.
Some things haven't changed much with today's tractors. Lights are still iffy, loaders themselves are about the same, and the 3 point hitch has actually regressed; that is, today's 3pt hitches aren't as sophisticated as they once were. PTOs are little changed; the most critical item there still being a separate PTO clutch so that the PTO can be worked completely independently of the tractor's motion.
40 HP and 4000 lbs have been mentioned as minimums. I'd agree. That is also roughly the dividing line between hobby and larger machines. I wouldn't go smaller at all, and would not be afraid to go a little larger. Stability and comfort go both go up quickly when you get to the 50 hp and over class. And stability is the absolute number one thing on your list. You are going to be surprised at how large you have to go to get decent sidehill stability just for traveling along without the complication of trying to do off-camber work at all.
Some things to note: Few people stay with the tractor that they buy originally, but there seems no other way to get the experience. So be sure to consider resale. By this time next year you are going to know a LOT more about tractors, and that isn't very long from now. As for New versus Used, don't automatically assume it needs to be new. These things last. Tractors are not at all like mileage-based automobiles in the way they wear. Guys (and a gal or two) who enjoy turning wrenches will definitely feel comfortable with a decently maintained used tractor. They know that any of today's major brands of tractor - moderately well-maintained - will probably last a lifetime or longer. And that with very minimal repairs. A few years ago I went through the motor and tranny on an older JD for a collector. I had full expectations of doing a normal rebuilt on wear surfaces, but was astonished to find that the wear inside was negligible. This was on a machine that worked hard and had a pretty tough life for decades. But the truth is that real repairs on any tractor are rare. You'll get more comfortable with that in a few years. It's not at all unusual for a person's first tractor to be brand new and his next one to be low hour used.
Or your second machine might not be a traditional tractor at all. Ours isn't. On our current place we knew we weren't farmers. We were expecting to do lots of tree and stream maintenance, a bunch of building things like rock walls & roads. And a fair amount of just lifting, carrying, & moving dirt around. Living in the mountains stability is key. So after looking at and trying tractors with add-on loaders and 3 point backhoes we went in a different direction and bought a machine actaully made for loader and backhoe work in rough terrain - one of Kubota's TLB line. 60 hp and 8000 lbs. Downsideis it was rather expensive. Upside is how nice and easily everything works and the stability is awesome.
For us there is still a little bit of true tractor type 3pt large garden work that we need to do like mowing, seeding, fertilizing, racking and blading.....for that type of surface work on an acre or two a little used 18 hp 4wd compact tractor is the perfect complimentary machine. It's handy, and used implements for it are very inexpensive.
good luck,
rScotty