Not doing the chain is a waste ive done several of these engines, yeah you can do the upper guide rails but whats causing the noise is the chain has stretched and then the noise is the rails getting smacked, to do the chain is approx. 4-5 hours, pay a Mercedes shop to do the chain if you cant, its easy
basically you connect old to new and roll it in, the lower guide rails aren't necc but now with the age I would do those, but then its a major job, but the piece of mind, my relatives 420 sel hit over 500,000 with just upper rails and chain at 220 k