Well, I drive a 1981 300SD (one of the models suggested above for this usage) and I do around 5-600 miles weekly. Got the car at 225K miles, am sitting on 242K now. Have prolly not spent more than a few hundred dollars for the last year of driving, repairs wise.

I am extremely biased, but I would think that, if you can afford to have wear items dealt with, that one of these cars would actually be a great choice. My car runs B99 biodiesel when I can get it (about 25% of the time) with no probs, cruises on 70-75 with reserve power, and does 25-27 mpg. I am inclined to think of the 126 chassis (1981-85) 300SD as a great choice for any kind of use aside from racing... just a fantastic comfortable ride, has "presence", and sips fuel compared to most cars its size. Almost anything else feels plasticky after driving a W126.

The 616/617 engine was designed for taxi use, there's no harm in using it like that if you maintain her by the book!