My '89 300E is a wonderful car, but cost me dearly in the first year. The AC compressor died; after 10,000 miles the sluggish transmission went south; worn valve guides needed replacement; the sunroof broke a track (expensive); several vacuum diphragms in the heater box (generally called servos) failed (8 hours labor to remove the dash to the firewall and 8 more to put it back together, MB seems to build the car around the heater box); fan belt tensioner shock replaced; the cruise control has never worked (the switch is bad, no telling what else). I suspect some of this might have been avoided with better care by the first owner and the dealer, who did virtually all service before I bought it.
With all that, I still expect this car will cost no more per mile after 3 years than my last Honda. The seats are a revelation. It will take twisting roads better than my old 280Z ever did, and a 14-hour drive leaves me feeling much less fatigued than it has in any other car I've had.
All this may be the best argument yet for spending $100-200 for a careful inspection before you buy. Ironically, the dealer, with expensive electronic diagnostic gear independent shops rarely have, may be the best place for this.