"Early M104 engines had head gasket problems. Also, all 1993-1995 Mercedes have bad engine wiring harnesses. Last, M104 engines have an electronic throttle actuator which is notorious for failing. All three items are expensive to fix. "

all M104 had headgasket problems that were fixed with the replacements from mercedes.

In fact Workshops have generaly seen the new M104 more for a head gasket change than the old ones.
But they all wear out and leak oil with age until the replacement is done.

There are no specifix bad engine wiring harnesses, all mercedes from 1993 models with both the new gasoline and diesel engines all got this new biological brakedown wire insulation, but some engine harnness are prone to wear out much quicker due to higher temperature fluctuations.
As is a huge problem on the 6 and 8 cyl. engines as they have the ignition coils directly on top of the engine.
And it is not only the engine harnness, it is used in the whole car as well, but is no problem here as the temperature are not as high as in the engine bay.

The reason for the quiter engine, was largely as an resault of the new type of tensioner chain guide.
The 1 type was made of aluminium with a plastic rail on top of it, were the chain ran.
The aluminium guide was hold in place of the chain tensioner that were also in metal, so the chain sound from the fricktion on the plastic rail was easily transfert trough to the housing of the engine.
With the 2 gen. guiderail as also fits on the 1 gen. M104 the aluminium base was replaced with some rubber/plastic frame that did not transfer the chains fricktion sounds through out to the housing.

The other chain guide/lubrcation housing was not an isue of noice, but an isue of getting the chain lubricated on both sides.
Where on the new type you have an oilspray nossle doing the same job without a housing, but then you lose the tothwhell and get an ekstra chain guide witch in fact makes more noice as the chain runs along this and in the housing it dident.