But it's common for human to focus on the details and forget on the bigger picture - that's the marketing effect has on us 
It's also very common for some humans to tell others what is right and wrong, neglecting that besides math there might be several right solutions for one topic.
And jeah currently there are indeed some good deals around. But as it's the goal to have a build from scratch, they might be not the fitting choice in this case.
For the question of the handlebar.. if you are shouting for infernal solution at the end, but are unsure about the dimensions and see willingly to have a test phase you can start with a semi integrated setup. Like having a "common" stem and handlebar and the hoses are entering the frame at the dust cap. Once you are fine and want to shift to an infernal routing, you "only" need to reroute the hydraulic hoses through the integrated handlebar. And you need to change the dust cap.
For that you will most likely need to cut the olive barbs unfortunately.. so you need some cm free length.
And this approach has one weakness: the dimensions of stems, handlebars, integrated Cockpits are sometimes not identic, even if they come with the same printed numbers. But that's a part of the game.

Is there the second fork to have an external routing possible? I think with the semi integrated dust caps (also available from FSA, Acros....) you could think about saving that. This is really working out pretty fine, if you want to use the frame with a common stem and handlebar. I have built 2 frames like this, and you can just use the infernal fork and are fine.