There are several types of rotors made for the W124. Some cross drilled and slotted rotors cannot be 'turned ' or milled down to give a new level surface and must be replaced when damaged or warped (red arrow).
Inspect the rotor for damage and wear. This rotor has a small lip developing on the other edge (as they all do as they wear). If you are going to measure the rotor for thickness make sure your measuring device clears the ridge so as to not give you a false reading. Check the rotor surface for cracks or deep grooves or ridges (yellow arrow). If any cracks are found the rotor should be immediately replaced. Light surface rust like that shown is not a problem and will come off the first time you apply the brake.
The rotor is held to the hub by two roll pins (yellow arrows) and a 4mm Allen screw (red arrow). This is a little over kill as the wheel studs actually hold the rotor to the hub once the wheel is installed and torqued.
Remove the 4mm Allen (red arrow). You will probably have to hold the rotor from turning and if you live in a four season climate it is a good idea to loosen this before you remove the caliper. In extreme cases you may need an impact driver to remove it.
You can now remove the rotor from the hub. You may have to give it a few taps with a rubber mallet to get it to break loose. Always hit the rotor on the bell or hat surface (red arrow) and not the surface of the rotor.
Before installing the new rotor make sure to clean the mounting flange on the hub so that it gives a good flat and even surface to sit against (red arrow). Installation is the reverse of removal. If you have opened the brake lines do not forget to bleed the brakes before driving the vehicle.