As I understand the replacement head gaskets have a slight re-design that gives them a longer life span than the originals.
Other faults/characteristics of the M104 engine are as follows;
There is another oil leak issue on the M104 engine, albeit a small one, they often leak from the front left hand side of the engine. This is normally the timing cover seal thats a complicated seal that never worked very well from the outset. To replace it costs about £70....thats labour + seal, but its best done by an Indi who has done loads of them as they know exactly how to get a tight fit on the seal.
Another common problem on the M104 engine is the infamous engine surge they can suffer from....especially noticable when the engine is fully warmed up with the car in gear but stationary....the engine starts surging and the car rocks back and forth against the brakes. This is caused by a dirty throttle body, which is rarely cleaned during servicing. The resulting crud that builds up on the butterfly flap in the throttle body creates an air leak which then causes over fuelling/confusion at the ECU.
Lastly, if the engine just dies on you, especially as you decelerate its likely to be the OVP relay, that again had a design fault from new. The internal soldering fails intermittently causing this problem. Another clue to its failure is if you try to start the car before the engine has fully warmed up....it can take ages to get the car to fire. Replacement is a 10 minute job and a new OVP costs @ £60........if your car is still on its original relay it only a matter of time before it fails...gauaranteed. The originals were made in Austria the replacements have made in Hungary on them.
The wiring harness has already been mentioned....to clarify this is effects 93 model year cars through to 95.
Personally the M104 is one of my favourite Mercedes engines...great sound once you pass 4000rpm and far for characterful than the later V6's in my opinion....my favourite Mercedes engine being the M119 5.0 V8