I have 10+ acres of fields that require now-and-again mowing (nobody needs the hay any more, so I just mow to keep it from reverting to woods). I use a '54 Ford NAA (31 PTO horsepower) tractor with a 5 foot bush hog. It has plenty of power for this, and will do a lot very quickly. A bush hog will do a pretty neat job even on lawns when set low, but still can get into the rough stuff - very versatile, but pretty power hungry. Any one of the family of small Fords starting with the 9N in 1939 would do that job nicely, as would many other farm tractors. The Ford (and similar Ferguson) are a good size, though. Avoid tractors with "tricycle" front ends. You can sometimes find a real bargain on these old tractors if you don't mind the looks, but some, such as the NAA, are getting a bit rare and collectible, so the prices are going up. Many of these tractors (including most older Fords and Fergies) also do not have a "live" PTO, which means that they run through the transmission and the clutch. An overrunning clutch on the PTO shaft is an absolute must if you run a bush hog.
For other attachments, if you have a long dirt driveway, a scraper blade on the back, or a heavy duty York rake (or both) are well worth looking for. The scraper blade can also plow light snow.
I think a good solid old tractor with a PTO-driven mower will serve better and probably be a better deal than a self-powered mower behind a Jeep or ATV. I've seen some of those in use, and although they do work well for big lawns, they don't have the grunt for thick and heavy fields.or brush.
As ChiXJeff suggests, one good possibility would be a small diesel tractor with a loader. I have an ~82 Ford 1510 (not for sale, mind you -just for example) which will do just about everything you're looking for, though its 20 horsepower requires a certain amount of patience - no bushhogging up hill in third gear like the old NAA. Some of those older Japanese-made 4WD tractors are starting to show up at reasonable prices. I got mine with a broken PTO but just about everything else in good shape, including a good loader and a heavy-duty rear scraper, for $2500.
Once you have a loader, you'll wonder how you ever lived without it.