I owned a 93' 400SEL, and it was quite reliable. I loved the styling, but that fact that I was always afraid something was going to break and bankrupt me, is what made me sell it. It was a great car, with an AMAZING sound system, huge amounts of interior room, and a powerful engine. It never missed a beat, cruised easily at 130MPH, and looked great doing it.
The one you are looking at appears to be in outstanding condition, certainly not as nice as mine. I would look at it, and if everything checks out and you like the way the car drives, then I would go for it. 5k miles a year isn't much, and it should be able to easily do that. One thing I notice, though, is that the trunk doesn't appear to be closed all the way, this indicates that the door-close assist system isn't functioning...a common problem.
The big-ticket items on these cars that are known to fail are the AC evaporator, electronic throttle actuator, Bose stereo amplifier, and apparently they go through suspension bushings rather quickly, but I never had that problem.
Good luck.