Sooner or later, you'll need a head gasket. When that time comes be very sure that a real MB tech does the work.
Other than that, the 88-91 300-SEL cars are very sturdy. Change fluids and filters on a regular basis and the car will last a long time.
I've had mine for 6 years and other than the head gasket, have had to do nothing more than routine maint.
Not bad for a 12 yr. old car.