I agree with Jackd. You likely have a leaky horseshoe gasket between the top front cover/distributor housring and the main front cover. It's a pretty quick and easy fix compared to pulling the head. I agree further that if you have the head off, have a full valve job done.

The timing chains in MB in-line engines don't have inflection points and tend to last much longer than the timing chains in MB V8s. You don't hear much about M103 chains breaking on well maintained engines. Heck, even on abused engines.

To add to dkveuro's list, if you have to raise the engine to work on the lower chain, consider new engine mounts and shocks (if W124s have separate engine shocks ).

Sixto
95 S420
87 300SDL