Hi Kestas:
I can't say for sure about an E320, but I can offer a comment on my 190e 2.6 When I had my head gasket repair done at the dealership ( I have a way awesome mechanic who takes car of my car), I had mentioned some problems with cold starts in the past - the car would shutter and shake before it would smooth out. For this reason, he decided to replace them for me.
If the E320 is the same as my 190e, my mechanic said that the valve stem seals can be replaced without removing the head on the engine. It's the valve guides that require removal of the head.
I consulted my service invoice from that visit, and the valve stem seals and a valve gasket set came to about $35. So I guess while you're in there, what's another $35, eh?
Hope this is somewhat helpful. Good luck with your car!
Peace-
Michael.