Funny you should mention that, one of my clients has an '82 300CD (turbo diesel) with just over 300k miles that he just took out of storage. He said his mechanic had the head off a year ago to do head gasket & there was no visible wear, cylinder wall wear was about halfway from new specs to recommended rebore/rering measurement - figured it would go to 500,000 before overhaul. Not sure about turbo lifespan but I've heard it's very good unless overheated. Some of the mid-80s 5cyls were abominations though, with the cylinder bores actually warping around 70k. I think those are now the subject of a class action lawsuit against Mercedes in the US.
-Chris