Common issues that you`re going to come across, are mostly with the early Mercedes R129s. Because the straight six suffers from the dreaded head gasket leak, quickly followed by a degrading wiring loom. The Mercedes V8s suffer too, but from the throttle body wiring causing rough running as well as ASR problems.
Battery drain is another problem usually down to a malfunctioning old alarm. Central locking too is fairly common failure. There could be issues also with the soft top not working if the module underneath the rear seat gets spiked by a jump start. Also, please check the electric windows are working.
But the most expensive issue with a Mercedes R129 will be the exhaust, the catalytic converters actually breaking-up inside the exhaust. This then causes a rattle if the vehicle is ticking over or even revved up.
Now, the gearboxes are pretty robust items if there`s been regular oil changes. The Mercedes four speed gearbox is probably the more solid, where as the later electronic five speed gearbox does suffer with the electrical plate that`s inside the gearbox. Oil has been known to seep through into the gearbox wiring loom into the electric plugs, thus causing a complete failure of the gearbox, and the inability to select any gears. This could be eliminated just by the regular oil and filter changes at 35 to 40,000 miles.
Suspension components do take a greater strain with the Mercedes V8s.This is due to the dramatic increase in the weight of the car. Actually it`s the front lower ball joints and the top suspension strut mountings that take pretty much most of the vehicles weight. There can be the occasional broken road spring on the front suspension, this is due to the rust build up and it can be a problem. Also, do please check over the rear brace support bars, these are starting to fail regularly on Mot tests when the underbody is checked, – these have been known to break sometimes.