I am considering buying a Mercedes-Benz 300D diesel, probably post-1980. Can you advise me on the best to buy?
Think about a W123 series 300D built after 1981. It is a five-cylinder and will give you great service. Little goes wrong with them as long as they are serviced as recommended, and the rest of the drivetrain and chassis is equally durable. Later W124 diesels are six cylinders and are also worth considering, but cost more. They perform better than the earlier cars but have more electronics and other modern ancillaries to give problems.