I am new to this board, but have had a few MB cars in the past. Currently have two diesel models: a 76 240 with over 545K miles and a 78 300 with unknown mileage (I feel it is around 200K).
The 76 has had no problems to speak of, except for the replacement of the trans. at around 400K and new floor pans. Rust has been a major problem, but the previous owner lived near the Atlantic Ocean in upper New England (I don't think they ever washed her). I recently had the entire body restored.... She looks show-room new.
The 78 300 was purchased from a local MB mechanic. The car is in great shape mechannically, but could use a little TLC on the interior.
Overall, I am very pleased with them and will never own another American car.

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76 240D - 545K miles
78 300D - ?K miles