Ok, I am betting it is the same as my 560SEL, but I am not sure if there is some upgrade to the 560 coupes in this area. My CD Rom does not say so, but you never know.
I replaced the upper guides and timing chain at 125K miles. They were really brittle, the color of a beer bottle. I was told the lower guides, down near the crankshaft, never go so not to worry. Also, pulling the front cover to replace them is a huge job, requiring removing the front head bolts. And removing the front head bolts risks pulling threads out of the Aluminum block, requiring removal of the heads to helicoil the holes and fix this. Big job I had no desire to perform.
You are right about the hole below the cam gear; it has an uncanny way of sucking parts into it! I strongly recommend stuffing a clean rag in there the second you remove the valve cover!
Anyway, I snuck the upper chain guides out by removing the cam gear while the chain was tie wrapped to the gear. Be careful not to let the chain drop far enough to get off the crank gear while you are doing it. I think I posted something about my work, replacing the chain, rails and cam gears. Poke around the search under my name.
Also, you will need a way to remove the rail pins. I fabricated a tool with a couple of M8 or M6 bolts, I forget which thread. I used them to pull the dowel pins. You may wish to invest in a couple of these before you start; I butched the threads on one of mine and had to wait until Monday when the MB dealer opened to grab a new one and finish the job.
There is a way to check the chain for stretch. You line up the timing marks on the crank at zero, and see where your timing marks on each camshaft ends up. There is a way to correlate the distance from the mark on the shaft to the mark on the head to degrees of stretch. This tells you how many degrees of stretch you have, and there is something in the manual about what is tolerable. Not sure what it is, (it has been a year or so since I did it), but I know mine were right on the money after the new chain, tensioner and rails were done.
I hope this helped you out. Let me know if I can answer anything for you. My grubby paws have been in there, so I might offer some insight. Good luck!
1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)