As a general rule, the last model years of a model are always the best. This gives the manufacturer a few years to "iron out" the new model bugs. While this is true with most american cars, most notably GM (1980 X-body cars, Cadillac's V864 and Olds 5.7 diesel engines) all which after a few years became better. I dont think MB ever had this problem. The earlier 1970s 300D's are simplier regarding their climate control and vacuum systems but were less luxerious overall and therefore less desirable. The 1982-85 models were turbodiesels. More power, same economy.
A non-turbo charged 4-speed manual may save you some big repair bills in the future because of its relative simplicity compared to a turbo automatic, but the later models are definitely more livable and pleasing motorcars to drive.