I have a 95 E420. Had the same expensive and bad advice from a Mercedes dealer a year or two ago. I think they remove a lot of engine compartment stuff and do it the long way. I now have an excellent independent mechanic who said that you'll know when a chain needs replacing when there is a rattle sound when you first start the car each day, or upon startup right after an oil change. He said that he opens up the chain area, connects the new chain to the old, advances the chain (not sure how) through the path until the old one is out of the engine, then connects the two ends of the new chain. He said that a new tensioner should be installed too. Said the part are approx. $250 and $200 of labor.