R&B: Check to see if the original Launch button housing is intact. Make sure the hat trick system in the back box is intact with all three plates (although these can be easily replaced now at Marco). Check to make sure the Nell log table has all the parts (motor can at least be replaced, but you still need the large plastic gear and pully wheel).
Beyond that, look for typical wear & tear that might devalue the machine like broken plastics, especially the lower lane guard rails. Those are often in sad shape on used machine, and you can't replace the original design of them (though newer ones by Stern are still made and work in place of them).
Lastly, make DAMNED sure the plastic ramps are in good working order. Those are almost impossible to replace. Once in a great while, one or two ramps will show up, but it takes about once a year, and some people know this and try to charge a ridiculous amount like $500 for them. I got lucky in that I found a pristine NOS full set for 50 bucks each in Norway, but that's obviously the exception to the norm.