Getting the right pad composition is the first step in conquering squeeking brakes. The face of the rotor can often aquire the results of overheating and pad degredation.
As mentioned mB doesn't recommend turning and the discs are so cheap you will have a hard time getting them turned much cheaper. If you do not have a run-out or thickness problem the squeek can be handled by rough sanding the surface layer and OE pads.
PBR "Delux" not "Metal master" pads will usually do the job but preparing the surface is the solution.
Bosch Master, ASE Master, L1
33 years MB technician