Jack summed it up pretty nicely.
The only thing I've noticed that is a chronic problem with the E420 is power steering pump leaks. The engine bay is pretty tight with the V8, and the extra heat bakes the seals in the pump.
Most people just rebuild the pump and replace the seals should the leak get bad.
I think the leaks that Jack mentions on the side of the engine are valve cover gasket leaks, but then again, any V8 will eventually need valve cover gaskets.
The a/c issue seems to be with leaky evaporator cores, but the problem was much more prevalant in the early 300E's in the late 1980's.
There are other issues with the a/c, primarily with vacum actuated flaps that are located behind the dash, but you'd probably replace these when the dash is removed to get to the evaporator.
Last, Mercedes V8's seem to be sensitive to timing chain wear, so inspect at 100,000 miles, along with the timing chain tensioner.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".