I've had an '84 300D turbo for three years now. I bought it from the original owner, with all maintenance records and window sticker (sold for 34K in 84, all options except heated seats).
I love it. Its still pretty solid, has some rust in the rear wheel wells. I'm planning on a Maaco makeover next Spring, with new sheet metal where needed and paint.
The good side:
These cars do run for a long time. I drive mine almost daily and it has needed a water pump & new brake pads (shortly after purchase) and new exhaust from the headers back due to rust (earlier this year). I use normal diesel.
Also, it is a Benz and looks great, and these cars have a huge fanbase. They are built like a tank as well, and you can feel it. Brakes are phenomenal, not just for a 22 year old car, for any car. My windows, locks and sunroof all work. Huge trunk, will hold a Rover 3.5 L V8-see pics!
The bad:
It is a Benz. Parts can be had from discount sources but are still expensive. They are reliable but when they go wrong you will feel it in your pocketbook. I'm not a DIY guy, even on my MG, so its a factor to consider. The performance is great once you are up to speed and moving in a straight line. It is a bit of a porpoise around town. The automatic climate control is a PITA. Mine has never worked so I live without AC.
Summary: A great car to live with if you don't mind the pokey handling and paying for proper repairs when necessary. You will smelllike diesel when you fill up, and for some reason, all diesel pumps are abysmally messy so watch you shoes.