My .02I have heard many things about galvanized stuff, and have read a lot about it in the process of building my new smoker, and I guess my advice would be not to risk it. If you go to any steel shop, they would probably sell you some scrap pieces that would work fine for dirt cheap. I made a smoke daddy type smoke generator out of raw steel, and I bought a piece of 4" pipe, several sections of 1" pipe, and a 3'x4' sheet of steel for under $20. That is pretty cheap for piece of mind.
I also considered that i will be serving food to my 3 year old, whose little body can take much less of any chemical contamination than an adult could. Even though it probably would never happen, I'd feel pretty awful if i made my kid or anyone else sick because i didn't want to spend the extra dough to do it right.
building a smoker is a lot of fun, and you are going to have to spend some money to build it anyway. why not kick a few more dollars in and do it right. Actually, even buying sheets of aluminum isn't really going to be that expensive if you buy scraps and cut ends from a metal shop.
Good luck with the build