There really isn't anyway to test the regulator except as a unit with the alternator. As such, one always takes a risk when just installing a regulator/brush pack.
Since a new/rebuilt unit will include a regulator this is where the risk comes in. I would suggest removing the regulator, examine the brushes, BUT pay close attention to the commutator ring where the brushes ride. If they are grooved heavily the risk is too great. The armature is probably only usefull for two sets of brushes and the second will probably last 50% less long.
Bosch Master, ASE Master, L1
33 years MB technician