John Deere 455 with 48 or 54 inch deck. The 455 is a stout tractor built from the mid-90s to the early 2000s. They don't bring the "collector" prices of earlier models like the before-mentioned 318s, and have a liquid cooled diesel that sips fuel, has more power than you'll need with a small deck and is very, very reliable. If you're lucky, you might even find one with 4 wheel steering. I have one (2 wheel steering) with a MC-519 collection trailer. I got it used about 6 years ago and wouldn't hesitate to tackle the job of mowing 3 or 4 acres a day for the foreseeable future with it.