The model to buy is a '83 - '85 300D Turbodiesel. It's a four door sedan with a five cylinder 120 HP turbodiesel. A/C, Power steering, brakes etc were all standard. These are comfortable cars to drive, efficient size and will deliver about 24 or 25 mpg in normal commute driving, more on the freeway.
The engine is all cast iron and very durable; that means its life expectancy is between 250K and 300K miles if reasonably well maintained, less if the owner has skimped on the oil changes. These engines are old technology, there are no computers etc. BUT the other side of this coin is that the engine's valves ought to be adjusted every 15K miles, preferably before the winter cold arrives. The maintenance schedule is: engine oil and filter change at 5K miles, valve adjustment at 15K miles and air and fuel filters and transmission fluid and filter at 30K miles.
The definitive test of any diesel's health and therefore remaining useful life is a compression test. Diesels are compression engines - the compression is about 21:1 and that heats the air in the cylinders sufficiently to ignite the fuel. So if the engine is worn the compression will be less and the engine will be harder and harder to start, particularly in the winter. So before passing any cash have the compression checked by an independent M-B shop as part of a prepurchase inspection.
Buy the best quality car you can, don't buy a run down car that's a "bargain". There are NO bargains only expensive repairs.
The cars are understandable to someone who has some automotive repair experience, C-D ROMs and M-B paper manuals can be found on ebay etc. If you buy one of these there are several on-line parts sources for a DIY owner.
Come back with specific questions when you find your car.