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When spending this kind of money on a bike, you have to get exactly what you want, otherwise its a pointless exercise. You wont be selling this bike on at any point, so dont worry about it fitting other people. This is your dream bike, you have to have it built how you want it, and not how others tell you to have it.
^This. Design by committee is very rarely a good thing.
Know what you want and allow the frame-builder to help you achieve that.
BareNecessities-
Totally agree.
My seeking help is largely on geometry given that I'm new to it and know many on here have experience through having builds.
Regardless of some people's dislike for certain brands such as Enve and people's feeling that hydro discs are not necessary, I like those things and will, ultimately, go ahead regardless.
xDOMx
I got an ISP on my Talbot (RIP). Purely because it looks bad ass, and we wanted to try something new. I have no issues with it. A bit of a pain if you travel a with the bike a lot, but since I ride a 54cm frame, it will still fit in to most boxes if you remove the ISP topper and saddle (no harder than taking out a seapost and saddle).
If you want one, get one. When spending this kind of money on a bike, you have to get exactly what you want, otherwise its a pointless exercise. You wont be selling this bike on at any point, so dont worry about it fitting other people. This is your dream bike, you have to have it built how you want it, and not how others tell you to have it.
Same theory applies to fancy expensive groupset/wheels/finishing kit. I got DA 9070 groupset, Enve cockpit, Reynolds wheels and fancy Fizik saddle etc, on my bike, which was completely ridiculous. Way beyond my needs, and stupidly expensive. But it's what i wanted, and what the bike deserved, so I sucked it up, and stopped being bothered about the cost. This should be a once in a lifetime purchase, so make it count!
As far as paint, if i were you, I would be fairly open with it. Choose a few colours you like, and supply a few images of past builds hes done that you like. That will be the best way of getting a great job out of the painter. It's kinda like getting a tattoo, if you have a very specific set out design, the tattoo artist wont really be into it, and you'll get an average tattoo, if you pick an artist who's style you really like, tell them the kind of thing you want, and let them handle the rest, you will get a great tattoo, as you are letting the artist have full creative input..... The same theory applies to getting a frame painted in my book.
Good luck with the build. Sounds ace. Hope the wait isnt too painful!