What galah ^ said about bringing a battery and cheap film. Consider it sacrificial film. Actually the batteries for this camera (it takes two) are fairly expensive, but there's no getting around that -- unless the seller is including a battery grip, in which case you can get by with 4 AAs, which are cheaper.

To check for the mirror motor gear problem, shoot through the entire roll of film. Around the halfway point, take off the lens and keep going, observing the mirror going up and down. Use various manual speeds. If the camera makes it to the end of the roll, you're probably okay.

It's worth checking. I've been burned on this same issue twice, with ZX-/MZ- series bodies.

A little background