Dave,
The doublewalled unit I have has 1/4" as the inner wall and approx 1/16" for the outer wall. The doors, upper and lower, are the thinner metal and not insulated. If I could, I'd change that to be an inner and outer 1/16" insulated door.
My cooker has the stack in the back corner and one of the things I don't like about it is the fact that it makes it next to impossible to have the same amount of smoke at the top shelf as I have at the bottom shelf. I've struggled with smoke penetration because of this. If I could weld, which I can't, I'd notch the shelves and create an extendable stack using threaded 3" pipe and unions. Each extension when attached to the previous extension would end just past the bottom of its associated shelf.
One of the features it has that is handy is the ball valve to drain the huge water pan. So if the water pan can't be removed, mine can't, having the ball valve is a must.
Your's is a sweet looking unit. Good luck with any upgrades you decide to implement.
By the way, by unit is a bit over 5' tall and weighs in around 625. It's like rolling around a loaded gun safe.