The 6 cylinder engines did not have this problem. I have seen engines with original chains go over 300k miles on the 6.
On the other subject, sorry for the failure. The dealer/factory will not help in this matter. They would argue the car went well over 100k mile with no major problems.
I would get an independent to look at it and give an estimate.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS