My opinion on plugs is to go with whatever mercedes puts in there from the factory.
Int he case of my 1992 300Ce, it has regular good ol copper core plugs (I think)
I know they aren't platinums, and I personally think my car runs great on the copper plugs.
Why change something like the plugs to a different kind if everything runs fine the way it is?
I would say go with the standard bosch plugs for you car. You should be able to order them from this site on the fastlane or partsshop.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.