The thing about doing the CLS wheels on a W203 C-Class or R170 SLK-Class is that the cars will use different types of lug bolts. The CLS uses 14x1.5 lug bolts and your W203 & R170 use 12x1.5 lug bolts. If you use your lug bolts on the CLS wheel the lug seats will not match up and there is a good chance that the lugs will fail and a wheel will come off while driving. Offset wise the CLS is lower than the C and SLK so they will sit very aggressive and you will at least need fender rolling, hopefully not cutting and shaving.