Welcome aboard! 1) I would put the thread in other smoker and grill designs. 2) As mentioned above 16 ga will work for the interior of the CC 3) I'm not sure on the dual fuel. I don't really fool with propane but Pete could probably help you on that.
Figure you'll need 1.5 Btu min / cu in of CC volume for propane. If you make your air intakes as low as you can, if the burner happens to blow out the propane can escape. An opening at the bottom would be better but not if you're doing charcoal as well. Chimney can simply be a rectangular vent at the top/sides or a short stack. I like the side vent as it won't allow any condensate dripping on your food. I only have 6 sq in and it's plenty for my 17,000 cu in CC.