This is a good thread with good advice. GaryCrowell has it pegged.
I have been farming 100 acre farm for 3 years. 50 acres of fields. I have disced, planted hay, cut and baled hay, planted vegetables, and done a lot of mowing. I square bale, not round, 30 acres twice a year.
I have been doing it with a 50Hp 4wd compact utility. I have enough horsepower. Not enough tractor. With weight, and filled tires I have about 5500 pounds of tractor. I am looking at a full size tractor, with 75hp ( engine, not PTO). I could get by with 60hp with the implements I use but I want some reserve.
Consider the following:
1. Terrain. I have rolling terrain and some hills. 4wd is really needed. Plus, the heavier front axle is MUCH better for loader work.
2. Wheelbase - longer wheelbase is more stable, better for mowing and traveling across fields when working. My wheelbsse is 71.5" and at 5mph while cutting hay the bounce gets annoying very quickly. When I spend 4 hours at a time cutting, raking, or tedding hay it kills me. On the other hand, around the barn it's great because I have good manuverability.
3. Tractor weight - Farming, and on rolling terrain, more is better, to a point. Moving round bales on side inclines, it helps. The ride will be smoother. Pulling loaded trailers and wagons - smaller tractors like mine do fine on level ground pulling a loaded hay wagon. BUT: a loaded hay wagon for me is 5,000 pounds, as much as the tractor and it will push the tractor down any substantial hill and is dangerous and can kill you or at least ruin your day, tractor, and underwear. While I think 10,000 pounds of tractor is too much, 7,000+ is what my farm needs.
4. Cabs are nice. Especially if you are sitting on the tractor for long periods. However, it is a pain in the neck when you are getting on and off all the time. A sun shade is often the better approach and will cost a lot less.
5. Current vs future needs. You will do well with 45-60hp and 4wd. If you think you will want to bale ( round bale) then you need to look at hp and balers carefully. A few round balers can get by on 45hp at the PTO but most need a lot more, and a 75hp utility tractor is needed.
We will keep our CUT because it gets a lot done efficiently, but we will be getting a larger tractor for hay. You should look at the specifcations for the Farmall 45-55 and compare others to it as it is a good size tractor ( i am not actually advocating this machine, but I know that it's size and loader capacity are excellent - I have no idea what they cost).
Last, what is your budget? I think the best value is going to be Kioti, but it really depends on your dealer. Look at recent used tractors. A common tractor here it the NH TN75 which is maybe more than you want, but is a very good tractor.
Let us know as you get closer!!