Welcome aboard!! I prefer the RF over a traditional offset because of the even temperatures vs the hotter temp zones in the traditional. I try to keep the stack above eye level to keep the smoke out of my face so I would go with the taller stack. The 105% ratio is right on the money so I wouldn't change that. You can go either way with cutting the door, I wouldn't be worried about 3/8'' x 18'' pipe distorting when you cut it.
Something to consider, till you put in the baffle plate it it is going to be super tight to fit 2 shelves in an 18 inch pipe. If your cooking needs only require 1, it is a wonderful size for a patio smoker. A second rack would only be about 8-10 inches wide and give you spacing of 4 inches max---great for ribs tho so I'd still put a slide in just in case.
Welcome to the crew. And yes, BigT's got you covered.
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