It looks like Mercedes recommends changing the spark plugs every 50K miles, so it looks like I am about ready. What plugs do you use?

Buy genuine MB plugs either at the dealer or through an online supplier. Maybe not the cheapest but affordable once every 50K miles.

Yes agree, they come pre -indexed, beware of counterfeit though.

Agree 100%. I purchased mine from local dealer. With the right tools it took me about an hour to replace all 8 plugs on the E550.

Other reputable sources include FCPEuro, just make sure to choose Genuine Mercedes as the brand.

I bought Bosch 96346, aka ZR6SII3320, from rockauto, $7.50ea three years ago. Just checked and they're $8.43. These are, from what I've read, the oem plugs minus the MB part # which saves a ton of $.
I had 58k on it and they made zero difference in power or mpg, which I expected.
My coils were dying left n right since they apparently have a timer to fail shortly after warranty, so I only changed the plugs since I was in there.

I agree with the others about being wary of counterfeit crap. Frikkin bass-turds, why don't they real job >: | So that means no Amazon, ebay etc. I hope they don't infiltrate to legit sellers, but who knows. I inspected mine and they looked legit, but how would one know? Also, would it actually matter? I also wonder if they bother making counterfeit Bosch, or do they only target the more expensive MB plugs? I dunno, just thinking out loud.

Then there's the popular one step colder plug option that is likely a good idea if it's tuned, or you drive it hard. And/or maybe better anyway and possibly less likely to be counterfeited? My notes say NGK 97506 and NGK 1555 were one step colder options, but you should research the subject because I don't even know what the differences between the two plugs are. I didn't bother because I'm running ~50% Ethanol.

Don't forget, some need to be indexed, which is very important.