The 1980 Mercedes 300D is a fantastic car. The engine is very sound and free of problems, provided you take some necessary steps. The in-line fuel filter should be transparent, this allows the driver to see any buildup that may accumulate from dirty fuel. This clogged filter may cause the car to lose power and and stumble while driving. This stumbling may appear like a huge engine problem and some repair shops may lead you into the premise that there is a major problem when its just a two dollar filter.
Some people may think that the "pings" and "knocks" are a nuisance, these can be remedied by using a great product called RED LINE this fuel additive provides lubrication for the top of the engine which reduces the pings and knocks and provides better gas mileage.
The pockets on the front doors should have some type of support inserted inside of them, because most people accidentally kick and crack these convenient hard plastic pockets. A small dictionary will generally provide enough resistance to prevent cracking.
Be sure to read the manual because you will find many secrets in the car. Location of drain holes. Manual sun roof controls if the motor burns out. and many other neat little secrets.