The brush assembly is replaceable, but I wouldn't put one in until I was sure that the connector is making proper contact. Use an ohmmeter to ensure that the terminals in the alternator plug are making good contact. I have had the wire come loose from the main terminal. It sounds as if you've already used a voltmeter at the battery to ensure that the alternator is not charging. The rectangular cover will easily pry off of the alternator plug to check the terminals. After you're sure that the connector is okay, it is probably time for an alternator. If after the regualtor/brush assembly is removed and you find that the brushes are gone, it may be worth trying a replacement assembly if the slipring on the rotor is not very worn.
If you do replace the alternator, my experience has been that you get more alternator miles per dollar with a Bosch authorized rebuilt alternator. The ones you get at your local parts house just don't seem to work for very long.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in