I have overseen the repair of at least 200 single cam MB V8 chain failures (3.5/4.5, 3.8/5.0, 4.2/5.6).
Here is the story; The number one cause of failure (9 to 1 atleast)is the fracture of petrified chain rails. The rails in the upper head do not normally touch the chain. When they are new they are opaque white plastic. They are indestructable. After 70-100k they turn brown and become as brittle as glass.
Now comes the sweet part; even with various tentioners and modifications MB could never 100% not loose the oil pressure in the tentioner. They just rattle every once in a while. Its a shame they never put a ratcheting tentioner in, but I'm sure they had their reasons. This rattle (you can't prevent it) breaks the old rails like glass and the pieces run through the meat grinder bending all the valves in the left head. Just like clockwork.
My recommendation is new rails every 60-80k. It is only about a three hour job and the rails are less than $5 a piece. Do a chain if you want but it won't fail on its own. After 150k you will have about 10deg retard in the passenger side cam due to stretch but we used to use offset keys to tune that out. This is factory method they couldn't have been too worried, originally. Thus my total recommendation is a chain before 200k and as many rails as necessary to keep them fresh.
Now for the second life. The bottom right side rail rides on the chain and will wear through at around 300k; so if you are in the neiborhood with head work or chain repair take that into consideration. This rail requires removal of the front timing cover about an extra 12-15 hours work with a head job. About 20 hours or more on engine with the heads on. We charge 40 hours to do heads and front timing cover on 107 SLs where the subframe and oil pan must be removed.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician