FWIW, alternators have slip rings, not commutators.

The bearings on a failing device will sound and feel rough if spun by hand. You can also grab the shaft and give it a wiggle and a tug. If you have ANY play, the bearings are toast.

There is no reason to ever replace the stator unless it has failed. It is nothing more than a coil of wire wrapped on an iron core. There is no "wear" occurring to it. It is either good or bad.

Check the condition of your slip rings when you have it apart, if they're grooved, replace the rotor. Resist the urge to sand the slip rings. If you feel like cleaning them, use Acetone and a clean rag. Nothing else. They should not have any sort of lubrication on them, the graphite in the carbon brushes in the voltage regulator is all the lubrication required.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]