Anything prior to '86 in generally the sweet-spot for MB diesels as the 6-cylinder that followed the 5-cylinder was prone to it's head developing cracks. The 5-cylinder 3.0L diesel is just a joy to work on as you couldn't get much simpler. It's like the Ford Falcon of German cars! And you will see a lot of high mileages on the ones that were well maintained. I've currently got an '80 300SD (W116 chassis) that has almost 600K miles on the clock and runs strong still with the original engine. The couple I purchased it from had been running it for years on soy oil they bought in bulk from a local discount center, which made it run much quieter until I started blending regular diesel back into it then the normal diesel clatter returned after the soy ran out.

Just remember to stick to the turbo cars for the extra power and economy. Actually, if you don't mind a little body and interior work I'm looking to part with an '82 300D Turbo that's been converted to 4-speed manual. It's very quick and an absolute blast to drive! If interested shoot me a PM and I'll get you the details.