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For my 2007 GL450, I bought spark plugs from MB dealer just to make sure (about $15 each). They are NGK plugs made in Japan with Mercedes logo on it.
Haven't replaced the coils yet.
MB factory uses Delphi so I'd recommend using that brand. Doesn't matter where you get them.
I got some branded coils, I think Bosch, and peeled off the sticker to find "Delphi" underneath.
Some coil-on-plug systems are "active", meaning they have circuitry to modulate the spark. In the MB system this is all done in the ECU. The coils are very simple, yet they do fail.