The first step is generally one of many. Good luck with the build.
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
That'll make a fine smoker! I wouldn't recommend the gas assist unless you have someone that is familiar with all of the required safety features because if anything goes wrong these things can become a bomb Doors 1) Pro the traditional doesn't require a lot of extra work to build and you don't have to buy a lot of extra material. Con the door is very heavy so it needs a counter weight to assist opening and closing. Also you get a face full of smoke every time you open the door. 2) cabinet pro easy to open and close, no face full of smoke because you aren't leaning over them. Con requires extra work and material. They swing out so you can't have anything on a shelf in front of them and if you have a damper between the CC and WC you have to make sure the control handle doesn't interfere with the door opening all of the way. I would go with the cabinet style doors if you are considering them, I believe they are worth the extra investment. If you decide to go that route there's a calculator at the top of the forum that Pete designed that'll help you figure the extra volume in the CC so you can adjust the FB size.
8" will give you plenty of spacing for the larger butts and briskets but for ribs and chicken you could get by with 4''. You could put your rack guides every 4'' and then adjust the racks depending on what you're cooking. A large turkey or a pig would be the main things that would need plenty of head room.