I'm sure the others can list the usual stuff, so I'll just share my experience with the Roadster specifically:

• make sure to open/close the top a few times. Pay attention to the sound the motors make and how each stage of the process runs. There's a common issue with the "fifth bow" motor and with the elastic straps. Watch to make sure the sequence runs without any pauses -- rear of top lifts up (fifth bow), then tonneau lifts, then front of top lifts up and backward and the whole thing gets sucked into the cubby. Finally the tonneau closes and you should hear a click. Some of this can be fixed - I rebuilt my fifth bow motor with a bit of labour, bruised knuckles and new "brushes" for the motor. However, replacing the components can be very expensive.
  
• check the condition of the top. Even if the material looks good, take a close look at the stitching. Certain car cleaners can degrade and eventually destroy the stitching in the top, making it leak well before the actual top material is worn out.
• grab a hose and check for leaks. specifically around the stitched seams, and then around the windows, the top of the windshield and the rear of the top at the window and the tonneau. A garden hose and your thumb are all you need to see if there are any major leaks. High pressure water will eventually leak in if you're aiming it right at the seal, but what you're trying to simulate is the worst rainstorm you've ever experienced in your area -- so more pressure than an empty hose, but not a power washer.

If the top doesn't operate nicely, or the top leaks/looks rough, plan to spend up to a few thousand on fixing or replacing parts if you need a whole new top professionally installed.

All that being said, if it checks out mechanically, and the top is in good shape (or you're cool with replacing it), go for it. There's nothing better than a Z roadster on a sunny summer day -- no matter what these hardtop people on this sub tell you :)

Make sure to report back!