My '77 W123 300D in dark green with the 'bamboo' interior remains my favorite car to this day. Purchased with 130k miles in '87, I drove it another 250k before an inattentive driver ended it. Reliable, easy and relatively inexpensive to maintain, the weakest link were the plastic vacuum lines and rubber fittings. I miss that car.
My step-father bought a 1960 180b, gas, and our relatives thought he was crazy. 4 cylinder, 4 on the column, no radio or AC, crank windows, 13" wheels, but we turned the speedometer 3 times and never a rattle. A great car and easy to work on.
Those cars were slow but tough, my cousins were on a road trip in the early 80s and got caught in a high speed crash in a 76 240D, they walked away with no injuries despite totaling the car. I also just saw one yesterday done up a a Gambler 500 car in prototype style dazzle camo with a roof rack and a light mount made out of a snowboard deck.
I had a 240D for a while. I was coming off a couple of high speed rollover accidents, and I looked at the Mercedes as penance for bad behavior. I learned that there were quite a few people who were impressed by the three-pointed-star without being interested enough to know that the car I was driving was a decade older than they thought. I sold it after a few months for a 40% profit to a guy who was having license-points issues that he attributed to his Mustang GT 5.0, and then I bought something that didn't merely gather speed.
I have had two 240D's and although they are quiet slow I find joy in driving them. I also have had a few 300D's and If I could find one with a 4 speed it would be my all time favorite I'm sure. The Mercedes Diesels are bullet proof and have never failed me .