Can ignition parts (plugs, wires, coils) "go bad" just sitting around? I want to do all new plugs, wires, and coils, but can't afford to buy it at one time, so I want to buy it in stages, but I want to install it all at the same time.

Probably not in the amount of time it will take you to buy everything you need. They probably sit on the shelves at stores longer than they'd sit in your garage. I really don't know if things can go bad, but I'd have to guess it would take a LONG time for something to happen, and it would depend on the conditions of where you are storing them (temperature, humidity, etc.)

It would take abuse ...I can't see them going bad other than dryrot on the wires.