Thinking of buying an SL 300 J reg . Did these have a problem with rust or was it before the cost cutting. The bodywork seems to be in pretty good nick. Anything to look out for?
Should be rust free - check where the front bumper meets the wing, inside all of the wheel arches and the rear of the hard top where it meets the main body of the car when installed.
R129's are not known for rust, even those built 1999-2001 (despite what some people will tell you!)
A '91 car should pre-date the wiring loom problems I think. Head gasket can be an issue on the 6-cyls IIRC.
Make sure you're going to be happy with the performance - it's a heavy car and the 300 won't be quick. The 500 is a better bet IMHO, if you can find a nice one.
Incidentally, to be accurate it will be a 300SL. R129s didn't become "SL..." until 1994.