Personally, I wouldn't consider a new one. Having said that, my experience has been that buying one from an owner/user, estate sale, etc. is better than buying one that may have been canabalized from different tractors by a "tinkerer."
Make sure the mowing deck is not too beat up or rusted and that it sounds ok and runs when you engage it. The rear end should not "scream" or be too loud, and you don't want smoke when you start or stop the engine. Oil is going to look dirty so that wouldn't affect my decision unless it really looks scary. Plan to change it no matter what.
Chris