The R129 SL models are very robust cars but had weaknesses like all older cars especially if they havn't been looked after. look for evidence that money has been spent over the years to keep any car up to scratch.I would be looking at cylinder head gasket, engine wiring loom, softop and rollover bar condition and operation in particular, plus all the electrical gismos the ABS and ASR systems. It pays to do your research on these cars first before buying--there are lots of posts by expert owners on the forum about this if you do a search using say SL and R129.