Did you measure voltage with the engine running?
12V nominal is okay with the engine off, but you should get a reading of 13.5 - 14.4V with no accessories on. Anything less and your alternator is not doing its job effectively.
For an early model like yours, you should just be able to replace the brushes, not the whole alternator...
...interesting that you mention needing to constantly work on the 240D but not the 300E...a lot of 300E "fix-me" threads pop up here daily, but most of the forum contends that the 240D is probably one of the most reliable MB models ever made.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car