What size of materials will you be cutting up? (figure that it's also the unintentional stuff that you could encounter)
Rocks? If you're going to be hitting a lot of rocks (in rocky areas it's a given) then you you'll want a heavy duty deck (deck thickness is an important factor).
As nickmags notes, probably a good idea to go with a slip clutch (though they can be a bit of a pain). I really like my PTO shaft from Bare Co as it also has shaft protectors that can release and slide back for easier connecting of the shaft (and also for maintenance).
This does remind me that I need to take my slip clutch apart and clean it up as it ain't slipping. Something that is essential to do is to make sure you can get it to slip; with shear-pins there's no such maintenance (either are intact or they ain't- and when they ain't it's when you're far from the shop and are in a hurry to get mowing done!).
I picked up a well-beat Bush Hog 286 for $800. At nearly 1k lbs (new) they were deemed "Medium Duty." 10ga decking and a 110hp gearbox. Seller used it commercially; he had to weld on an additional 1/4" plate steel to the top of the deck as the deck was getting beat up pretty bad (rocks). Blades were new. I put about $500 into it (new PTO shaft and some repairs to tailwheel bracket) and it works great (though not pretty). New units like this run a minimum of around $2,500 (closer to $3,000). Not a lot to bush hogs, in which case buying used shouldn't be anything to shy away from.
