Shanta, early 500E's had 500SL brakes (300mm ) and your late E500 has 600SL brakes (320mm). The upgrade for you is to add 600SL *rear* brakes, which will bump you from 278x24, I think, to 300x20.
ItalianBenz, you're right, bigger IS better - but you have to figure out... at what cost is it worth it? Is it worth (for example) $4k to go a half inch larger? I doubt it. Less weight is always good though.
Why don't you check out the Brembo kit from Bekkers, they have the lightweight calipers you want, with the same 13.6in rotor size as the C32? I believe that kit fits W124's, why not give them a call & find out.
Omegabenz, I'm with you - what is the size of the 2000-up SL500 brakes, and is that front only or front & rear?