Re: buying a tractor....
Yes, for the most part they are categorized by hp which suggests the amount of implement it will drive, how much weight it will balance on a three-point hitch, etc. You'll find that hp is not the true limiting factor and most tractors will have more than enough for whatever you want to do. I used a 5' brush hog on a John Deere 650, which is a compact/mid 18hp utility tractor. Aside from having to hang 200 lbs of weights on the front to prevent wheelies, it did just fine in the power department. What I'm saying is, size is the more important factor. A new 35hp tractor might be the same size/weight of an older 20hp tractor, but don't let that be your determining factor. They'll all get the work done, you just may have to do it a little slower. I spent a lot of time on a 7000-lb John Deere G from the 40s that only made 20 hp from a monster 413 ci 2-cylinder engine. It would easily drag an 8' brush hog or a 5-bottom plow, you just did it in a lower gear. A comparable replacement (size wise) today might have 60 hp.
If you're buying new (which I don't necessarily suggest), my top picks are Kubota, Mahindra, and New Holland. John Deere is still a fine choice, but they have taken a few steps toward being an assembler rather than a manufacturer. That's not a bad thing, but some of their compact/mid tractors aren't proving to be quite as reliable as their former glory. If you buy used, it's a big world out there. Late 70s-mid 80s Ford tractors (pre new holland era) don't command big bucks (because they don't have the new Holland name on them) but they are true workhorses. Dad has a 1978 Ford 1900 4wd. It really works hard and takes abuse. It was a municipal tractor so its first 3000 hours were abuse, and we have since put another 2000 hours on it of heavy hydraulic use (backhoe and loader) and it will not die. This can be said for most tractors. They are so remarkably overbuilt. Dad and I have tractors dating back to the 1930s and they still get used on the farm for real work. No idea how many hours are on them. Possibly tens of thousands with almost no repairs.
60s-70s Massey Ferguson 135 is a good bet, but they're hard to find in 4wd
Daddy just bought a New Holland Boomer 45 that is just chub-worthy, but it was new and with the loader and options it was nearly $40k.
Whatever you get, please make sure it has safety gear intact and has ROPS (roll bar) and a seat belt. It doesn't seem that necessary at 3 mph, but I've rolled tractors before and it's the only thing that saved my life. You're not going fast so it feels safe and stable, but the torque they make (along with massive gear reductions) means that one mistake can send you flying. The first one I rolled was just user error. We were loading an old Ford 8N and forgot ramps for the trailer. I got the brilliant idea to chain the axle to the foundation of the barn and use 1st gear to crawl the front up in a wheelie, then back the trailer under it and crawl the back wheels up on the trailer. All went well until one of the back tires popped back down causing me to pop the clutch. This lurched the other tire up on the trailer, wheelied the front, and it landed on its side on the roll bar. If the guy I was buying it from hadn't fabbed up that roll bar, I might not be typing this right now. This is, of course, a case of sheer stupidity on my part, but I had driven 500 miles to buy this thing and I wasn't going to let a lack of ramps stop me. Tractors' abilities make you want to try stupid things.
Another suggestion if you're going to be keeping some trees: Pay the extra for the folding roll bar. They fold down in the middle and still provide a little bit of protection, but they get out of the way for tree branches or low garage doors.
Around here, you can find some used gems at auction sales, but if the Amish show up, they always take the high bid. Yesterdays Tractors magazine is a good resource. They have a forum with some nice folks and a classifieds section. Craigslist is fair for tractors around here, but good old fashioned newspaper classifieds still turn up diamonds.