I am trying to re my timing belt in my 1985 300D Turbo 4 door sedan, but the 300CD, 300TDT, 300TD designations are confusing me totally

Your car is a 300D Turbodiesel or a 300DT abbreviated. A 2 door coupe is a 300CD. A wagon is a 300TD. A wagon with a Turbodiesel is a 300TDT.
Randy D.

I never knew a Mercedes Benz had a timing belt.

Your car doesn't have a timing belt, MB diesels have chains. Your car is a W123 chassis which came in 3 bodies, 4 door "D", 2 door "CD" and wagon "TD". The "T" in "TD" stands for touring, as in when the family goes for a "tour" of the country, they take this type of vehicle. Starting in 1982 the cars had turbos on them.