my alternator had (obviously) had the brushes replaced in it before, and had at least 40K on it with new brushes. if you make sure there isn't excessive play in the mechanicals, yeah, go ahead and change the regulator. i'd suggest taking the regulator out and ordering it by part number from bma, instead of relying on them to match a unit up for you...here's some background
http://www.bimmer.info/forum/showthread.php?t=1222
might also consider extricating the alternator and taking it to a hard-core auto electric rebuilder (one that doesn't cater to the general public for the most part)....with parts all over the floor, grease everywhere, shelves full of used alternator parts.....i got quotes for $50 to take mine apart and replace bearings and brushes, clean up &c. &c. yeah, you can just replace the regulator.