All 300E's had 65amp alternators I believe. A few months back I installed a 120amp alternator from an S320, and it dropped in with no issues. Just had to swap pulleys from the old alternator. Used stock wiring connectors (although I will upgrade to larger gauge wiring at a later date). They also make a 150a alternator in the same housing.
http://www.peachparts.com/shopforum/showthread.php?t=53236&highlight=alternator+upgrade
Here is a thread with more info on that upgrade. It's a good one!