I had over 250,000 miles on my 400E with the M119 and never had an issue with the chain. My E420 probably has over 200,000 miles as well. Recently, just for peace of mind, I pulled the valve covers, pinned the exhaust cam sprocket with a drill bit and replaced the chain tensioner and upper chain guides, which were completely intact but brittle. My only regret was that I didn't use factory valve cover gaskets and now the passenger bank is slightly leaking at the front.

I'm curious how they checked for stretch. Did they remove the belt tensioner?

__________________
1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles