There are some threads on here with detailed pictures of regular steel barrels used for brewing. I'm not sure of the chemistry, some folks said the solubility of iron may not pose problem, but the pictures were scarier than snot.
Whatever material you go with make sure you get a bottom with a welded seam or one that's been formed. The cheap ones have rolled seams and they can leak after a few sessions, especially with the drastic temperature changes in brewing. My neighbor has 3 and 2 of them make a nasty pool on the ground but he's to cheap to replace 'em.
55gal for $169
RBI equipment on ebay (45gal for $168)