The regulators tend to fail before the alternator itself, or even the bearings. The regulators are typically very easy to replace, and so much cheaper that it may be worth just trying this first with no further debugging. On my 300E, I can do it without removing even the belt. Your 190E may differ.
Alas, my alternator did finally die. Probably could have replaced the brushes, bearings, etc., but time is money, so I bought a rebuilt unit for just $84.00 net (shipping and everything).
If you do decide for full replacement, I found that this vendor had better prices on rebuilt MB alternators than any salvage yard quote on a used unit, plus a 3 year warranty. 6 months later, I am still a very happy customer.