In my 99 S420, I had the same funky smell. Turned out my O2 sensors + MAF ruined the cats by making the car run rich. The excess fuel ends up showing its presence by giving the potent sulfur "rotten egg" smell. Twenty hour job according to my service advisor.
2 O2 Sensors
2 Catalytic Converters-$2500 a peice, Special Order parts
Maybe Parts if sh?t happens
Another cause could be faulty plugs, wires, distributor or even the fuel injection system.
It is not a normal smell at all. I have heard of V12s on this board that were running rich and ended up needing new engines. Once you redline you are pushing alot of fuel that has to be burned in a very short time.