The 500E M/C has a slightly larger piston for the front brakes, so it will move the brake bias forward. If your calipers are stock, I would not use the 500E M/C, stick with the correct M/C. Also, I believe the ports exit on opposite sides for those two cars (201.029 and 124.036), so the hydraulic lines will probably not bolt up at all.
FYI, details on the M/C & booster specs are in my brake spreadsheet:
http://www.w124performance.com/docs/mb/W124/124_brakes.xls
And, some photos of the 400E/500E booster & MC are in this thread:
http://www.peachparts.com/shopforum/showthread.php?t=209172
