Ok so everybody is not understanding what I am getting at! I don't know what size tire needs to go on a 20X8.5 rim to fit on my E420 without rubbing. I don't have any idea what it would actually call for. A 225/45/20 would go too, if they make a tire that size. I am asking if ANYBODY on this forum has any experience with 20's on a 210, or if anyone has even tried. That's what is being questioned, not obnoxious responses. I appreciate everybodies input, but your opinions aren't answering my questions. If someone has tried to bolt in 20's and they will not go, then so be it and I will get the 18's for 200 more dollars.
The biggest question is one no one can answer...because we need to know the rim specs. "20s" could mean anything, but the other key measurements are RIM WIDTH and OFFSET. There are 20s that will fit, but you will not have anything like a "touring" ride, unless you're driving on glass-smooth pavement. Otherwise your dentist will be a fan, because you'll be shaking your fillings out. You're also adding unsprung weight, and a larger rotational weight which will degrade your acceleration and, probably more noticeably, your braking.
18s are as large as you can go without seriously degrading ride quality, and even those add a layer of harshness. But if you want 20s, it's a "gotta have it" mental state, and it's your car, so go for it.
And if you want to know if the wheels you're looking at will fit, then post the full wheel specs as noted above. Without that there is no telling. Just looking at an R wheel, though, I thing the offset is wrong, so you'd end up dropping some serious cash on spacers and custom-length lug bolts.
Last but not least, posting specific questions about a particular car in the "general" forum is the best way to get the least amount of useful information; and then getting defensive about not getting the answer you want doesn't improve things, either. There is a forum specifically for your car, so hit the W210 forum. Heck, you might even read the wheels sticky and search around in there to gain a bit more knowledge.