rear discs look worn they messure .515'' how far before replacing

there are actually three specs for three different ML's: 11.4mm, 19.4mm, 23.4mm. what's more important is if there is an sharp edge lip on the outside edge of the rotor. no matter what thickness the rotor, if you install new pads on an old rotor with an edge lip, you are gonna have awful brake noise. suggest installing either Textar or Pagid brand pads and ATE or Brembo rotors; these companies make brakes for MB. turning or resurfacing rotors is not advised under any circumstance, per MB.