I would argue that MB inline 6 gas engines are very easy on timing chains and can go a long time without replacement if oil and filter changes are done religiously on time. The chain is running a short, simple oval loop and therefore doesn't see alot of wear. V-8's, OTOH, see a more tortuous path and have greater travel, hence more stretch over time.
If you really want to talk timing chain nightmare, find a shop to let you see the inside of a 600 V-12. Four cams, gears in contact with chain for only about 120 degrees of rotation, I'd hate to be paying to maintain one of those!
Also, on the point of mileage and engine speed, most MBs are built and geared to run at higher rpm's than American cars for a reason. They want the car in the power band when you're at normal speeds. Also, they're designed for that engine speed. I drove my '91 300TE (4matic) (if you're looking for a TE, these aren't for the faint of heart) from Maryalnd to Florida, and it saw 22 mpg at an average speed of 70 mph with a full load of people and luggage. It normally sees about 60 mph and gets 20-21 mpg. I was amazed but figure that's where it was designed to run. Also, a 300TE is a 4000 lb car with 177 hp (M103) or 216 hp (M104). You still have a good load to push when the car is empty.
Good luck, hope you find the right car for you!