Your criteria contradict themselves. M5/M6 are total gas guzzlers... I know from experience that a M5 is 100% capable of single digit mpg for city driving pretty consistently.
Plus I doubt you will be able to find one in good condition for under $30k. If you do it's probably due to extreme high mileage and maintenance will not be cheap...a new SMG clutch is like $5k and its only a matter of time before it gives out!
I can't recommend any of the mercs for the same reason plus they did not age well IMO from a styling standpoint.
I know it's not what you want to hear but if spending more than $30k is an issue then the inevitable multi thousand dollar maintenance bills and two fill-ups a week will put these cars beyond your means.
I'd say your best bet for $30k would be a mid mileage 335i sedan with a tune
Current Car: '13 SANTORINI BLUE E92///M3
--6MT/Speed Cloth/ZCP (ED Thread)
Former Cars:'10 E90 335i MSport
/ '07 Interlagos ///M5
/ '09 135i
/ '10 E93 335i
Last edited by Curt2000; 02-11-2013 at 10:45 PM.