likewise for my 1989 190e2.6, the benz service advisor recommended that i change the original water pump bypass valve assembly (and i think that required a change in the entire water pump) mainly because the old bypass assembly was made of plastic and they said that over time these tended to crack (!!!). The new one is made of aluminum.
i also had my differential overhauled, which is expensive. Mine is a 5speed manual, and because manual shifts are harsher on the drivetrain than an automatic, and because the engine braking of a manual also contributes to wear and tear, this could have contributed to the premature (or was it normal?) wear of the diff. If yours is an automatic, you might not have this problem, though differential gear whine is common and considered normal.
the M103 engines are also known for head gasket oil leaks near the firewall.
knock on zebrano wood, i hope this is all ...