You may be overkilling, but I'd need to see your aplication and arangement. There's no reason that 1/8" material cant hold 2 Bar if it's welded right. Brakets for the blower should be sturdy, but 1/4" 6000 aluminum should be more than enough.
Going with 1/2 is sure to tbe strong enough, but you're getting more expensive material than you need, and taking on more weight than is neccesary. You should have no trouble with 1/4" for your flanges, as well, as long and they're flat and you've got good bolt coverage .