Kent,

1995 was the last year for S-Class diesels called 300SD in 1992-1993 and S350 in 1994-1995.....S-class cars are great in any year.I would argue that the 140 is the most engineered benz ever.And probably the most expensive to maintain
As for the engine problems of the 1990-1995..its true...but if you get one thats already had the engine rebuilt/replaced by a dealer you should be fine for 500K.
I own a 1992 300SD that has had the engine replaced(warranty) at the MB dealer and therefore was put in to another 48/50 warranty. If you check with owners of these 1990-1995 diesels that had engine problems(and not all did ) you will find that once the engine was "repaired" with what amounts to beefier and modified parts there were no further problems....but I would advise as with any later S class(140) ..EXTEND THE WARRANTY!!!!...because after an evaporator core($3000) or a closing assist pump($1000) you will be glad you did...and these are very common repairs on the latter S-class cars...good luck