I wouldn't have it done at the dealer because they are enormously expensive. Do it at an independent Mercedes Benz Service and Repair facility that has a good reputation. If you don't know any, try using the search function in the good shops sub-forum.
An engine wiring harness should cost about $550 and take 2-3 hours to replace.
I had head gasket replaced and also decided to spend a few hundred dollars more to do a valve job - new guides and seals. They really weren't worn out, but I figure this is the last time in the next several hundred thousand miles that the head will be off. The new head gaskets have steel inserts and it should never leak oil again.
Another reason to at least do the valve guides is that while you can replace valve seals with the head on the block, the head has to be removed to do the guides so you're killing two birds with one stone.
My job cost about $1,400 for the gasket and valve job including parts and labor.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".