Make sure the basement doesn't flood. Pinball machines absolutely hate water. A dehumidifier would be good, so things don't get moldy. The temperature in a basement stays fairly constant so your probably good there. High temperature swings create condensation which electrical and metal components hate and cause flaking of backglass. Electrical outlets on the ceiling would be awesome. That way you don't have cable salad on the floor. Those are the first things that come to mind. Good luck!

Protect against lightning strikes. Even if the machines are off, as long as they are plugged in they are in danger. On main breaker/switch for the machines so you can turn all of them off at once.