There are plenty of possible issues with any car that age but those last 124s were phenomenal cars. My buddy who owns one of the best independent MB shops in SoCal drives a '94 E320 as his DD. The harnesses are a known problem as mentioned, there is no aftermarket part and a new one will run ~$900 if needed. They are an easy DIY install. When the head gaskets go, often they merely leak oil externally. Not a catastrophic failure by any means.
All of the other things Randy mentioned like sensors and MAFs are potential failures on any car that age. In my experience, often MB parts are cheaper than Toyota parts, especially if you are web-savvy. Of course, Toyota parts are outrageous...
I have nearly every part on that car available used; relays, trim, electrical, lights, you-name-it. I've been eBaying used MB parts both from my own stock and my friend's shop. Email or PM me for any used MB parts, that goes for anyone else reading this as well.
A sample:
