On my 2000 E320 Wagon I thought the battery had lots of life left but the E class seems to need a lot of juice to get the starter to engage.

New factory batteries are only about $125 and would be the first place I would look.

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2004 E320 4matic
2000 E320 wagon
-- in the past---
1992 190E 2.3 5-speed! (European Delivery Baby)
2000 ML320
1978 240D - 340k mi