Dwight:
Check out the following posts (there are many more in the archives from people who have done what you are contemplating).
500E brakes
500E brakes
What year is your 500E? From what I have gathered, the eary ones used aluminum caliper 500SL brakes, while the later ones used cast iron caliper 600sl brakes. The used price seemed to run about $1000 for the 600 SL calipers. As to putting the old 500E calipers/rotors on a 300E, some people indicated that there may be hub work needed to do this, and they won't fit under the stock 15" rims. 400E brakes just barely fit under the stock rims.
[Edited by JCE on 02-24-2001 at 01:49 AM]
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John
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E