I also buy junk machines to restore.
If I were a rich man with a row of machine titles then this would be my filter, in order. I would try to pick a few games per filter and eliminate the rest. Assuming all games are of equal condition :
1. Overall visual appeal based on cabinet style, colors, patterns.
2. Playability. Is there enough interests on the board to keep it interesting?
3. Game features such as drop and rotor targets, gobble holes etc.
4. If I still had a few to choose from and couldn't decide then I'd choose the one with a higher production run.