I agree with the voltage charge of about 12.9 volts at 2000RPM without everything on full blast. That seems about normal for my 190e and most 12V cars.
Its pretty easy to take out the alternator and check the brushes. My 190e alternator died completely as a result of worn out brushes...I had about 1/4 inch on one side and 1/2 inch on the other. I picked up brushes at a nearby retail parts store that oddly enough had the exact item that cost $40 plus tax. This was competitive with Fast Lane, and a little quicker, so I did that about a year ago with no problems since.
1985 - 190e 2.3L 168,000 miles
1986 - Pontiac Firebird, 111,000 miles, 305 4bbl