actually your not supposed to turn mercedes rotors. They recomend replacement. I bought some not long ago, I want to say maybe $50/side? If you are going to go through the trouble of removing them, you mine as well replace them and repack your bearings.

He should be able to measure them, I think the minimum thickness is ~12.9 It is stamped on the rotor itself, get a red shop towel and wipe the brake dust off and look very carefully. On the genuine benz rotors, it says on the top wide ridge, on the ate's its stamped on the thinner ridge.