Get a quote from the parts shop on this site. I have heard of people changing the brushes and regulator without removing the alternator, but they told me when they did mine it made it easier to take the alternator out as some of the screws were a little rusty and tough to get at. I think that will be the toughest part of this job. Given the age of the car I would not be surpirsed if maybe some are siezed up, but you may get lucky.
1995 E420 - retired
1986 420SEL - retired