I paid $738, for timing chain and tensioner replacement. I also had a quote of $1,100 to $1,200, for the same job plus rails. So, your quote of $350 is an incredible deal. One warning I've read on this forum or perhaps on the Mercedes Club forum is to be sure that a genuin Mercedes chain is installed. Apparently, after-market chains have been know to break shortly after installation.
Thanks for the oil tip. And, maybe that makes a case of synthetic oil with a thin viscosity that supposely reaches moving parts faster at start up. Also, the tensioner is partially activated by oil pressure. So, a faulty tensioner could result in a chain "slap" and broken rails.