I love my 1991 300CE. When looking for a new car, I zeroed in on this body style and tried to find the youngest well maintained example that I could afford. Bought it in June 2002, 112k, for $9500. This was a 100% dealer serviced car and the PO had two new MBs in his driveway so I skipped an inspection. Now have 127k and did just have to do the head gasket plus water pump. This is one of the 2 big repairs to be aware of. The other is the leaky evaporator that requires tearing apart the dash. I think the wire harness problem is with the mid 90's models, not with the 90-93 models. I also had the work done at Mr MB. As an aside he mentioned that the trannies tend to last to 150-175k but not a huge deal as rebuilts can be done for $1500-$1800.
Mine still shifts perfectly. Besides the gasket, all I've done are the front brakes, drag link and a few other minor repairs. I switched out a perfectly good original viscous clutch.

Around here, SoCal, there are plenty of 300CE's to choose from. I would insist on an inspection. After 100k, there are a lot of things that will begin to need attention. Go on an extended test drive. Be sure to turn into driveways and listen for any clunks from the front end. Any hint of oil scum in the coolant overflow tank means head gasket time.

glenmore
1991 300CE
1990 LS400