I have a 300D with 149k miles. I have turned all four of my original rotors twice and they are still well above minimum thickness. No unpleasant side effects have been encountered, except that the last set of pads do squeal a bit when cold.

That being said, the condition of the rotor before machining will determine how much metal needs to be removed, so every situation is different.

What "makes my day" about this topic is that countless individuals will insist that "it can't be done," but not a one of them can provide a credible reference to support their position. I have the official MB service manual for the W123; it contains no prohibition on rotor machining.