Arthur works on these cars for a living, so his advise should be considered.
If the voltage regulator is going out, you can check it 2 ways(in my opinion).
The first is to remove it and look at the brushes. Here's a photo of your VR:
http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1EQ0PTV2S1KQ0KZE7P&year=1991&make=MB&model=300-E-002&category=F&part=Voltage+Regulator
The brushes are 1/2 inch long when brand new. If they are the issue, then their length will be much less. These pop out the back of the alternator. 2 phillips screws hold them in. Very easy to get in & out. On your car it helps to elevate the front end in order to gain access. Ramps can be useful. There may be a large shroud underneath that must also be removed. 8mm socket gets the screws off.
Another approach is to connect a voltage ohm meter(VOM) to the battery. Set it to the 20 DC scale. Red lead to + battery terminal; black lead to - battery terminal, then start the car.
In my case when the motor is cold, I see 14.20 - 14.30, but my alt was replace about 1.5 yrs. ago. Before doing that, I would see 13.95 - 14.05 on a cold motor.
Once the engine warms up, the alt. output will diminish some and you won't see the voltages just listed. I now see 13.80 - 13.90 once warm. Before replacing the alt., it was getting down to 13.30 - 13.40 when warm.
If the brushes are really worn, I'd suspect that you'd see a low reading on the VOM.
My 2 cents.