I can highly recommend the 455. It meets all the requirements you listed. Perhaps my favorite feature is the three cylinder Yanmar diesel engine. I have just never been around a small engine with this kind of power. It has exceeded every expectation I had of it, and it still surprises me after nearly a year of ownership.
You can find a good used one for 4000 to 6000 depending on features, hours, and accessories. If you're looking for something a little older/smaller/less expensive, its predecessor, the 430 is a beast, as well. For a spark plug model, the 400, 420, and 318 get a lot of respect. For something bordering on antique that will still work itself harder than you can imagine, look for a 140 -- that will be the next addition to my collection. I saw a stock one out pull tractors 20 years newer and heavily modified at the county fair this summer.
For background info and an active forum on the subject, check out
Refreshing...... The classifieds there are probably the best marketplace for a clean used machine you could possibly find. Many of the regulars build museum pieces out of wrecks and sell them for enough to get started on the next one.
