When I replaced my timing rails I took care to measure how much of the 12 degree 'stretch' was from various components. Replacing the chain tensioner made the most difference - 3 degrees of the 12. The new upper rails/tensioner rail took out another 2 each side, so I was down to 5 degrees of true chain stretch. I opted to leave the chain in place (it is 100,000 miles old) until it stretched to 7-8 degrees (MB sells offset Woodsruff keys to compensate for up to 10 degrees of stretch, which suggests they can live with 12 degrees). The ignition timing in the 560 is electronic and adjusts for this (the earlier 116 engine - e.g. 380 - has manual timing adjusted by twisting the ignition distributor on its axis You do not need to remove the engine cover to do this job.

Much expert comment from various mechanics suggests a life of 350,000 miles for the lower rails - where you do have to remove the engine cover for replacement.

I documented the job and you can see how dramatically the plastic rails change color with age. I do not know if that is bad as I have not done before-after brittleness tests, but I constantly read they get brittle, a piece breaks, rides up the sprocket and forces the chain to jump a tooth, with expensive consequences.

You can change the tensioner without removing anything from the engine (use 50mm M8 bolts to start it in place - new ones are very tough to push in towards the block to get the shorter factory bolts started) but you do have to remove at least one cam cover to measure stretch - the method is illustrated on my site.

If you DIY, cost is modest - $100 for the tensioner and rails, etc and another $60 or so for the chain. Just running through a new chain is easy, but not necessarily the answer - the labor is in the rails. It took me three casual afternoons to do - much messing about with the dog during the process to keep my morale high when I got stuck!