Well ordering the parts should take a few days to a week or two to deliver based on how busy the company you ordered from is and replacing all parts should take about an hour or so maybe two if you've never done it before and need to look up a guide on how to do it. I learned from Fubarduck. Opening HRAPs can be confusing the first time. One thing to note about that particular stick example you have used is that is has been pre modded compared to other horis. It has the Seimitsu buttons not sure which ones though and those are actually what people put into their sticks when they mod.
Basically what I'm trying to say is its already got good parts in my opinion, unless you just aren't a fans of seimitsu (I have a sanwa stick and a seimitsu one) or you just want to change button colors.
As for assigning buttons most people I know who use sticks want to emulate the arcade feel as much as they can so that when we actually visit the arcade and play on a cab we don't need to rely on quick buttons. I'm sure you know everything can be done with A, B, C, and D (and taunt but not a lot of people really care about that button) so you only need four of the eight buttons. But i'm sure if thats what best suits you you could map the other four to fit your taste RCs, Barriers, Counter Assaults and whatnot.
Again as everyone else has been stating its up to you and what you are comfortable using and practicing with.
And as for the IAD Kara-can its easy in training mode but when put to actual use in real battles I tend to fail, the motion is still new and weird to me but it just requires a little work and i'm sure you'll get it.
*Edit: I've been around for a few months and this is barely my first post??... wow...