"Audi" and "college car" should on;y apply to kids of wealthy parents getting sent away to an Ivy-league school.
Any Audi your brother can afford will be high mileage and in need of maintenance. While that is true with ALL cars, maint and repairs on an Audi will be much more than something domestic or (shudder) japanese.
He wants good on gas and is going to school - I would suggest looking for any pontiac G3 left on a dealer's lot. Last time I heard, GM was throwing $6500 in on any leftover, and I can get him another grand or so off.
mid - 30's mpg and a full factory warranty would be a good thing for a student to have. A G3 is an ugly, utilitarian car - but would make the best sense. I dunno, maybe $7500 off of a 12 or 13K car is too expensive...