Set yourself a budget and keep looking - it will depend on if you want something that needs a bit of TLC or is great straight off the bat. If a machine has come straight off site it will need a good clean (say $30 for cleaner), new rubbers (around $35) and possibly replacement of a few plastics ($?). If the operator has been on the ball then it should be all working.
DMDs tend to cost around the $250 mark.
Personally, to date I have tended towards machines with good to very good condition playfields; saves a lot of work later. Plastics can be replaced cheaply enough, as can target decals; cabs can be redecalled if required; backglasses and translites can be found at a reasonable price, but replacement playfields are never cheap.
