Sounds like you are on your way to designing what you want. Kick back and let the folks here bounce your ideas around and you will get it figured out. They helped me stay on track. Oh....Welcome!
Welcome to the crew.
Having access to laser cutting, bending and welding equipment definitely puts you a step ahead. And as you say, you can always bribe someone to weld it up for you.
Depending on how big you want to go, but have you had a look at the GF mini? It is insulated (although the firebox door does get hot), small and you can get a fair bit of tucker in the cook chamber. Just a thought, but if you want to design your own thing, go for it. The folks here are very helpful and will give you pointers and help when / if you get stuck.
Learning generally boils down to "Repetition or the avoidance of pain", some people learn by doing, some by watching and some just have to pee on the electric fence.
Whether you think that you can, or that you can't, you are usually right. Henry Ford
Welcome aboard!! The calculator is for an offset, to my knowledge a calculator doesn't exist for a vertical smoker. I also would recommend taking a look at the GF smokers if you haven't already done so.
I've been there twice. G/f & I were there a few years back, car toured the southern half for 11 days (drank like fish, and still couldn't hold a candle to the locals.) We might be going back to Dublin in a year, but my job situation might be ruling that out.
I'd second the gravity fed recommendations at least as a consideration. You said a pedestrian gate..... how wide, and what sort of condition is the walk back to the yard? My double pan gravity fed weighs in around 500kg and really does not like anything except a very hard surface. Personally..... go heavier on your firebox if you can. Rock wool insulation in all of the cavities should make the exterior fairly safe, except for the firebox/ash door. Skin.... mine is skinned in 3.175mm stock, that's a lot heavier than necessary, but easier for a novice welder to work with. Probably not a concern if you're bribing someone with smoky goodness.
Oh, and Guinness or Murphy's?
"Measure with micrometer, mark with chalk, cut with fireaxe." Ted M., USCG (ret)
"Smoked pork. It belongs in me." Giles L