As the owner of 2 S-Class MB's, I'll give you some tips on what to look for. Check the operation of the climate control system in all its settings. If you are unfamiliar with the MB automatic climate control used during those years, read the owner's manual. It can be very expensive to repair if not working properly. Check the operation of all the windows, sunroof and power locks. Check the transmission does not leak. Make sure the engine idles smoothly. Remember to floor it during the test drive so that it takes off in 1st gear. V-8 loves to shift at the redline. Check under the mat in either side of the trunk. The rear window seal usually leaks with age and rust develops unseen under the mat. Check all around the door window rails where the weather strip is. Water usually accumulates there and the door frame rusts. Check under the hood where the A-Pillar is. Leaves and dirt accumulate there and if not cleaned for a long time the base of the A-pillar will rust. Of course, you won't know until is too late since it is in an area that is hardly seen. A broken dashboard or leather seats are ugly and expensive to replace. With regular maintenance, these vehicles are bulletproof but expensive to maintain. Take your time and find a nice example from the many that are out there.