Coupes and Convertibles:
First thing to do is to examine the underside carefully for signs of rust. Anything needing more than a little wire brush and Rustoleum would be a good reason to pass on purchasing the car.
Next check the glass for scratches or cracks.
Glass is probably the most expensive area to fix.See it it drives well with no squeaks or rattles. Get over 50 mph and verify that the torque converter lockup works and all gears are there. Watch out for any electrical issues, windows go up and down properly, remotes work. Radio, tape, and CD all work. ABS light comes on at startup and goes out shortly, run brake test.
Check for proper operation of the A/C. Check to see if the headlights go up and down with no extra whirring, popping or other noises after the headlight door has stopped moving.
Beyond that just check the normal things: maintenance history, when were the brakes flushed, trans and oil changed (with filters ?).
Interior should be on par with the value but no rips or tears. 16 way seats is a plus.
Basically look for a clean original car with no major appearance/mechanical items needed.
Easy rule of thumb:
- Rust or cracked glass = pass
- Nonoperational A/C = $ 1000 off
- ABS light on = $ 1000 off
- Transmission issues = $ 1,000 off
- Other mechanical items = same cost as to fix a 1990 Bonneville (even has the same ECM just a different PROM)
One item to check carefully on a convertible is the top weatherstripping. Replacements are not available anywhere. Look closely where the door glass contacts the weatherproof at the rear of the door as that is probably the most worn spot.
Make sure the header bow latches securely with no rattles.
Check carefully for cowl shake as some are horrible while others ride very nice.
Try to find a car that has been garaged most of its life as there will be better paint and likely a much better interior. Convertibles are also more leak prone so a garaged car will fare better.