Make sure you check for oil consumption. Look inside the air cleaner. If there is oil on the air cleaner element, beware. If the drip tube that runs from the oil separator in the bottom of the air cleaner back to the oil pan is leaking beware. If you pull off the oil filler cap while the engine is running and oil blows everywhere, beware. On top of all that, it wouldn't hurt at all to get a compression check done before buying it.

Also, I had a '77 240D way back when. The 240D (the engine is the same as a 300D, except it has one less cylinder) had a recall on timing chains and prechambers. Both were defective, and either could hand grenade the engine in the event of their failure. Be sure those pieces were upgraded.

Also, make sure the transmission doesn't leak and that it shifts well.

These are basically sound cars from which you can get a zillion miles, but you have to get one that is in good shape to start with.
Good luck,