I'm a little confused. I thought that the 140-series diesel used the 350 engine? If so, the problem I heard about was that the old 300 engine was bored out to accomodate bigger pistons, and that was the problem -- LOTS of trouble with cylinders becoming out of round, bent connecting rods, etc. People I've talked with say to run, as fast as you can, in the other direction rather than buy a 350 . . .
RE: the dreaded evaporator problem, having replaced mine on my '93 400E this summer, my tech said MB has gone back to COPPER, rather than aluminum, evaporators, due to the corrosion problem. He used an aftermarket copper one in my car.