Mercedes make mercedes engines end to end, some parts will of course be bought in (Turbo's are generally Garrett units).
However, Merc do sell allot of engines on to Chrysler and GM for other models (Chrysler Voyager comes to mind) - on the flips side, I believe that the A and B class 4 cyl engines are GM variants or collaborations .....but I could be wrong.
I guess the lorry engines are pretty generic too.