My 91 300SE just crossed the 140K mark yesterday. You can't go wrong for $3000 unless it was involved in a serious accident. One of the easiest non Diesels with power features to care for.
About the weakest point in the car is the engine. There's bound to be a head gasket leak or two if it's still original. I don't know when MB improved the valve seals on the M103 so a valve job might be due as well. If it has plastic radiator necks, keep an eye out for cracks. If the belt tensioner is set properly but still feels loose, it'll need a new tensioner. Motor mounts loose integrity over time so check those. Gee, I'm starting to go into basic wear and tear items so I'll stop here.