The only cars that Mercedes has ever recommend timing chain inspection are the 1984 to 1986 190e 2.3 & 1993 to 1995 diesels.
However inspection of the timing chain for excessive stretch should be done any time the valve cover(s) has been removed. Normal mileage for replacement seems to be around 140k miles!! The cars with the most frequent services last longest!! 300SEL or 420SEL the choice is yours. Sure hard to beat V8 power.
MERCEDES BENZ MASTER GUILD TECHNICIAN
ASE MASTER TECHNICIAN
27 YEARS M.B. TECH