This sites Fast Lane and a few other MB WEB providers show W9DCO as the std. plug. W9DCO and W9DC are the same plug except the W9DCO has a slightly wider electrode - might fire a tad bit better.
I use Auto Zone for tool rental..ie...cooling system pressure tester, etc. Beyond that - zip.
I wouldn't believe squat an Auto Zone parts person had to say about an MB or any other Euro make. Everytime I've ever been inside one, there was always someone at the counter returning a part that "didn't fit".
Buy your MB parts from someone who knows your car.