This is kind of a tough job to do if it is one of your first tries at repairing your own car. Accesability is difficult for the alternator on W123 cars. Look in the Factory Service Manual first. There are many links just do a search.

On the flip side, on many occasions just the brushes for the alternator have gone bad. You can solder in new brushes or you can buy a complete regulator that the brushes get soldered into and it will have the brushes already included. It is simple enough to pull the regulator out of the back of the alternator. There are two screws holding it in. Then you can look at the condition of the brushes to determine if they need to be replaced. However if your alternator belt is squealing it may mean that the bearings in the alternator are bad and you have to replace the bearings or the alternator. If you just do the brushes it should be less than $10, if you do the regulator it should be less than $30 and the alternator should be less than $150 for a rebuilt.

Good luck

__________________
Doug

1987 300TD x 3
2005 E320CDI