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Mercedes-Benz Fitting Chart and Wheel Fitment Guide | Alloy Wheels Direct
You probably want to stay near a 225/215 width. Should be 225/45/17 to match the stock w124 size. 215/50/17's would probably fit a little better and the extra sidewall would help the ride to be a little better.
With that offset/width of wheel you may need to do the fender spacer mod in the front and roll the rear fenders a bit. They will fit but it might take a little messaging. I've got staggered 17" AMG wheels from a CLK500 on my w124.