APB: Here's a link to my experiences with changing the timing chain on my E420. You need to pin the exhaust cam sprocket with a 6mm drill bit or you may have difficulty reinstalling the new tensioner. (Once the old tensioner comes out, it will allow the exhaust cam to rotate under spring pressure and tighten the chain.)
http://www.peachparts.com/shopforum/tech-help/304246-e420-m119-timing-chain-tensioner.html
I also changed the upper timing chain guides since they were cheap and very easy to replace (stuff paper towels around the chain to keep the broken plastic out of the oil pan).
The only other thing I would add is to make sure you get factory valve cover gaskets and not the crappy Victor Reinz brand that have a tendency to fall off the back of the cover as you are trying to install them. They also leaked on my car and a friend's 500E, who did his at the same time I did mine and used the same Victor Reinz gaskets. He replaced his with factory gaskets and the leaking was solved.
I have also read where guys over on the 500E board
********* have had the same trouble with Victor Reinz valve cover gaskets. The extra cost of factory gaskets is well worth it.