Take a long test drive in whatever you're interested in. Test every button, make sure it has 2 working keys, listen for noises, check the lights, if it's a convertible, operate the top. Make sure there are no warning lights on (CEL, ABS, etc...).
Look at any records the owner has and before you pull the trigger, get a car fax.
Mechanical parts are available and aren't outrageously priced for the most parts.
Body parts can be hard to find.
NOTE: Brake lights will not work without the key in the ignition. Neither will the horn unless the RCM has been refurbished. (Yes, we have a guy that does that).
Also the car will beep angrily at you unless you click the seat belt before starting it.

They are great cars and you can get a lot of support on this site.
Best of luck to you!