Minimum thickness for rotors on a ML320 are as follows; front 23.0mm and rear 13.0mm. This is for a stock rotor. Check to make sure your rotors are oem and not aftermarket. It's your call whether or not to replace the rotors at your 2nd brake change, but truefully most ML models don't make it to the second brake pad change before the rotors are below min specification. Actually I see quite often that when the pad light comes on for the front pads that the rotors are only about .5mm above min specification and I usually suggest replacement at that time. Whether or not I suggest replacement usually depends on when the next state inspection is due. It doesn't make any sense to just replace the pads if your state inspection is coming up and your rotors are getting close to min. spec. I've also seen some of these mercedes models have the rotors wear out before the pads do. Hope this helps
Dustin