There seem to be no head gasket leak problems with the M119 engine.
On this site, there are a few posts about engine wire harness problems, but they are limited to 1992 and maybe 1993 400E's (same engine and displacement, 4.2 liters).
The wire harness problem is nothing like the M104's.
I've never seen a black on black E420, but many Pearl Black with gray leather E420's.
One thing I've noticed though, on these cars is power steering pump leaks. It seems the extra heat from the engine is hard on the hoses and seals in the pump.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".