13 gage would really be enough I would think. But go thicker if you want. Or you could go thinner on the inside and thicker on the outside.
As for insulation, I would use a solid foam product. You can get it in 2" thickness. The fire / heat rating would be your main issue, not the food issue because it will be sandwiched in between the inner skin and the outter skin. Assuming that you use a 2" frame, all the welds should be far enough away from the insulation that that shouldn't be a problem.
I wouldn't drill any holes in the outter skin that aren't absolutely needed. That will just expose the insulation to the elements an have points of potential rust.
Hope this helps answer some of you questions and concerns. Of course, I'm not a pit builder, I'm a fire truck designer... so what do I know? 8-[
"Of all the imaginary friends I've had, I don't think there was one that I didn't end up having to kill."
in seach of Umami