With a 90 GXL, I would first take off the radiator cap and check for bad coolant seals. If the coolant doesn't bubble badly (after you have started the car) I would then replace the cap and drive it. If there are no noticeable problems, do a compression test afterwards to make sure both rotors have near same compression (takes 15 minutes). These cars either run great or don't run. I think a coolant seal test and compression test would reveal the 2 silent problems that FCs have. Anything else you would probably notice while driving.
Re: ???what to check before buying 1990 rx7 gxl???
the engine runs nicely when warm, but the engine needs to b reaplace soon and i have no idea where to look for a quality rebuilt for not that much $$ anyone have any ideas???