I used to build the Rayco Stump cutters when I worked at the factory. I also worked in the repair dept. and saw lots of damage. I would recommend a used one that has had documented maintence. These things get the snot beat out of them, and most users don't seem to care that they shelled out 10-30K on them.
What kind of stump grinder do you want? A walk behind? A smaller pusher type (kinda like a rototiller) They will wear you out just getting to the stump, a tow behind, a self propelled, a track machine? There are a ton of machines out there. If it is a belt drive system, check all the belts good. These need constant adjustment, where a hyd drive is a little better for adjustment. The main cutter drive belt (normally a gates poly chain belt) will stretch some, but not like the engine to jackshaft V belt. Hydraulic motors that get slammed into gear at full throttle are worn very quickly and costly to replace. This is just a start of things to think about. I'm sure some of the everyday users have a little more insight on the problems assosiated with certain units. Good luck
Steve