Do the timing chains and tensioners ALWAYS have to be replaced like on the older 420SEL motors (M116?) ?

What is the service interval, if any? I have searched here for hours and have found conflicting information. Some say 100k, some say 150k some say 200k or even that they never need to be changed - only if it gets loud and starts clacking around.

Why all the different advice?

It seems like the failing plastic rails/guides are the big issue. I am surprised people have not advised only to replace the rails, and meerly check the chain at rail//guide replacement.

Do ALL of the tensioners and guide rails need to be replaced?

Looking to buy an E420 and just want to be educated.

Thank you!