If I were you, I wouldn't do it. I clean the engine on my 97 E320 with a damp cloth. There is no reason to risk some electrical failure for beauty reasons. I don't even apply any of those engine shine products. They only lead to discoloration and become magnets for dust. I only rinse out the air tubes that lead to the air filter on my S420, S600 and 300E. The E320's air filter tubes aren't as accessible and I don't bother. Becareful of the powder coating blisters on the valve cover because they will chip off if rubbed too hard. I've never used MB's engine wax because I am told that aluminum doesn't rust.
Instead of washing the engine, open the air filter assembly and vaccum the debris collected in the air box. I do this every month.
If you engine is leaking oil, then the belly pan cover is probably covered in oil. I just spent 1k to fix the head gasket leak and the dealer washed the belly pan too.