Buy the game you want buddy. Fill your game room with games you like, screw what everyone else says. Best advice from this group, which you'll get, is don't get ripped off.
If you like a game that's not so "popular", you have a bargaining chip. Show up, and at least know how to inspect the machine and don't be afraid to pass on a BBH, it will pop up again.
You found this site, so I'm sure you knew that. Simple things to keep in mind - backglass keys are usually on a hook inside the coin door. So you unlock the coin door and another key is hanging there on a hook. Kinda near where the quarters would drop in if it was closed. Use that key to open the backglass lock that is usually in the center of top of the head of the game. The glass then slides out once you push up on it, so be careful.
Behind all that crap is all the circuit boards, and batteries...You'll have to move the light board for the lighting behind the backglass, it will have some latch to get it open.
Behind that baby, you then should see something that it looks like the inside of a computer. Look for battery acid, scorch marks, big ass blobs of solder. If you show up and at least know how to inspect the game, you're good to go.
Haggle a bit, but buy what you want. Another hint - If you see a dot after the credit display on the scoreboard, the system check is detecting an error. Some switch, coil, isn't working...time & date not set means dead batteries which leak. If a game sits, a switch will throw an error but if you activate it, the error clears. Probably dirty...
I'm rambling but I'm home alone finally, and the TV is broken. (and if I don't stop playing Black Hole, I won't go to bed.) You get the picture, if you don't want to fix that crap, pass on it.