Can you ear the starter motor rotate?
Do you have the impression that the starter "slips". The teeth of the gear may be worn also.

If you are sure it is that part of the starter the culprit, I would first give a try cleaning it. That part is just a kind of clutch (I did it very long time on other cars) where dirt can accumulate.

I leave to MB specialists to confirm

__________________
Keep us posted especially if your problem is solved
Present cars:
My car: E-class 420CDI, 2008, W211, V8, 180 000km

Wife's one: C-class 220CDI Sport Coupé, Euro, 2002, W203. 345 000km

Son's one: GLK class 220CDI, 2009, W204

Sold E class 260E, W124, 1988 beloved car sold after 489 000 kilometres of reliable services (engine M103, clutch and 5 speed manual gear box all original).

E-class, W210 320CDI, 2000[/B], 225 000km, Sold