The difference between mid-grade and premium is $0.10 per gallon around here. That's about $1.50 per tank. I don't think that's a very good place to try saving money when you've decided to invest in a Mercedes. Just pack a lunch to work once in a while and you're even.
I'm with Larry on the knock management functions. The system is there to help avoid catastrophic failure. Even the manual says not to run anything other than premium unless you have no choice, and then to go easy on it and put the proper fuel in at the first opportunity. The manual does not say "put in any fuel you want, we've got you covered".
There are many great (non Mercedes) cars out there if super-economical driving and great gas mileage are primary considerations.
Experience is something you don't get until just after you need it.