Joker, don't ya think a tank is just a little much....one thing about it no one would get in your way getting the Q. delivered!
Seriously though, there's a whole bunch of things to consider when building a smoker pit. My advice is to do a great deal of research and listen to a lot of different opinions, what others have built, etc. to gain a broad understanding. It'll serve you well in the long run. You don't want to invest your time, efforts and resources into something that could have worked better with even the slightest adjustments in building it. You have to have adequate venting, the exhausts have to be in good position. Use of baffles, plates, having good air-tight seals, ETC. We want this thing running really well for you once it's finished whatever thickness of steel or cooking area you chose to have.
A lot of the offset cookers use 1/4" metal but IMO that's not necessary. It just depends on how big of a build you want. What you want to do. Your preferences. The thicker the metal, the better it holds temperature but also takes more time to heat and the larger area you have you don't want to go to thin. I heard tell once that "metal doesn't lie" and it's true. The firebox it's important to have thicker steel there especially so I wouldn't go anything much less than 1/4". And one that is plenty big there's ways to calculate what's needed around a third or slightly more the area of your cooking chamber. I also like the idea of insulating a cooker. Particularly the firebox. Not necessary but in my opinion worth the effort. If you chose to do any insulating in between, you don't use as thick of metal of coarse.
For me personally, I have absolutely no reason to be making me a really great big huge cooker. I'd rather be spending some money on the area that I will actually be using implementing the use of insulation that helps for it running efficiently. A little more area than I think I might initially need so that I don't outgrow it too soon yet not so large as I'm wasting fuel for no good reason cooking with it. Something to consider anyway. You have many good options to choose from deciding your build.
I'm not at all an expert or anything about building barbecues. I've just did quite a bit of researching to familiarize myself in a bunch of different areas. Different ideas and various opinions. You'll find lots of helpful people here with important things to share with you.
Good luck and have fun! Welcome to Smoking Meat Forums. F.Y.I. you can officially introduce yourself in "Roll Call" it's a good way to say hello.
Forgive my "sense of humor" earlier. I bumped my head one time been a little off my rocker ever since....