Your chasis is a W140, and it might be interesting to do a search on this site under 'W140'.
You might also find useful a search on 'M104'. Your engine is the last year of the inline six cylinder engines made by Mercedes, and is an 'M104' engine. It is the same engine used in the E320's from 93-97, and a 2.8 liter version of this engine was used in the C280's from 94-97. In 1998, Mercedes went to a V6 engine in all models.
I know from memory (and experience) that oil leaking from the head gasket on these is very common, and Mercedes never seemed to figure it out. I'd budget $2,000 for a head gasket replacement.
You may get hits on your search for engine wiring harnesses being bad on these engines, but that was only from 93-95, so you're clear on that one.
Other things to consider on these engines is water pump replacement, mass air sensors, throttle actuators, O2 sensors.
Your W140 chasis is considered the most expensive Mercedes ever in ownership costs in terms of parts cost and maintenance. In particular there were problems on leaking a/c evaporators on the early models, so you may or may not be clear on this one. It seems the whole car was built around the evaporator, and the labor to change it is enormous.
So, while the car and engine may last forever, there are some expensive components that may go out, so I would get the longest extended warranty that is offered to me, and thank them for it, cause it's a good risk that you're gonna need it.
Believe me, the search function is useful, and anything you could possibly imagine has been discussed at length on your car.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".