Not sure if the one I'm looking at is working 100%, what typically fails & what should I look for on inspect?

Playfield wear around the large teal inserts and neutral zone hole.  Almost every game I have seen has broken ramps, broken plastics are not so bad as you can buy the set or almost all of them singularly.  GI plugs fried, all the playfield GI comes from 2 wires unless they have been replaced they are usually fried (no big drama to fix)
Play a few games and make sure it is working properly, STTNG has heaps of optos and you will notice if things aren't quite right as the game will not be able to find a ball or it will always play 2 balls etc.  That being said it is usually an easy fix anyway and a good starting point for negotiations.
Good luck.