return them as soon as possible, they are designed to fit older model mercs, not the W210. you need an ET between 30 to 42 for the W210. i'm surprised the guys selling you the rims didn't know that and stop you from buying these rims. if you fit them you WILL destroy your tyres as the car will bottom out on the rims on our roads. also you will destroy your wheels bearings as the leads you are going to be putting on them will be murder and they WILL fail.Well ... I have made the purchase, but I am still not so sure. I can give them back and get the typical AMG style in 17 with the standard ET and all.
And one more thing though, if I do go ahead with the deep dish what tire size options do I have? 215/55 might be a bit thin in my opinion. For a width of 7.5 can I go with 225/55 or 225/50?