Since it is an automatic, there is not a down side to a SMALL leak except for the mess. With a manual transmission you risk oil soaking the clutch disk.
On your car, unlike many MB's, the pan can be removed without removing the engine. This does make it feasible to replace the seal without removing the engine. After saying that, I now wonder if the M103/104 engines have a one piece rear main seal that would require removal of the transmission.
Maybe someone can chime in regarding whether these engines have a one piece or two piece seal.