if the car is in good condition and runs well, mileage like that really wont hurt it too much.
Things to look for are tires in good condition, no rust! that's important. I would look at the fluids to make sure they look and smell normal, do a blow-by test, there's a couple ways to do that.. make sure all the lights work etc. I would personally take it to a mercedes specialist and have them give it a once over to know what your getting into. Some things you can do simply other things if your a novice it's best to take it in so that you dont damage anything important. Oh, Another thing is, if the Diesel has a pre filter, (small clear filter located on the injector pump side towards the bottom of the engine near the frame rail) check that the Diesel is the right color, it's usually green or has another tinge of color, I think it's blue. look at the filter itself, if it's black or just in general really dirty you will want to investigate why, Diesel can grow bacteria, and algae depending on what it's exposed to but a dirty filter could just be a dirty filter, so it pays to investigate further. the search function on the forum here can give you a wealth of knowledge, There's also a lot of great knowledgeable members here who can answer specific questions about motors, types of cars etc,

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hum.....
1987 300TD 311,000M Stolen. Presumed destroyed