JCE, I just went through this with my euro 230E and decided to stick with the solids but I will come back to it in the future....
The vented rotors (same diameter for me) fitted well and the calipers also fit well, the problem for me was the brake proportioning valve and I think there is something called a meter gauge also. This part didn't seem that expensive from what I could tell for a W124 but I couldn't be bothered. The reason for the new valve is that the vented disk are thicker, I ended up warping the vented disk I got because I did not upgrade this part.
If your rotor has a smaller diameter you may also need the caliper mounting hardware or the caliper itself from the C280. The C280 caliper may compensate for the new diameter without having to change the mounting hardware.
Another thing you should be worried about is the venting of the brakes. You may need the c280 front bumper to give adequate venting to the rotors.
Hope this helps some...hopefully you can compare notes with somebody who has a c280, good luck