The "rule of thumb" on MB timing chains is to make the change every 100K
miles and/or 10 years.  Generally speaking the chains don't break but the
associated pieces with the chain is where the problems occur.  Some of these
parts are made of plastic and the mileage and heat tend to make them very
brittle.  They break, fall into the chain teeth, and that's when the major
problems begin.  And in your case MB had so many problems with the single
chains on the 380 models that they went back to a double chain.  As I recall
the last time I had a chain and related pieces replaced the cost was around
$1200.  Have it done at an independant MB shop.  They do these all the time
and are much cheaper than going  to the dealer.  That goes for all work on
older MBs.  The dealers don't even like to work on the older models.  New
cars are pretty much "plug and play", in one door, out the other, here's the
repair bill.  They don't and in many cases can't fix them like they used to.
Again, find a good independant shop and get to be best friends with them.