Don't buy a 40K car if you can't afford to spend an extra 20 cents a gallon on running it. As a fraction of the repair costs on an MB, it is insignificant anyway, right?
A mechanic I know (he has had an MB shop for 30 years in NY) swears by premium fuel - says if you open up an engine past 200K miles, the difference is very clear. Apart from the slight loss in performance and mpg, which evens out some of the price gap anyway.
Aviation fuel is 120 octane sometimes - heck, they could save a lot if they switched to 87!!