looking for an enabler
 

Sorry for the dumb question, but I have zero experience with Shimano. Give that, I need help: if I were to buy this frame, approximately how much would have I have to spend to get a complete build kit? I'm thinking Ultegra DI2, but I'm willing to be convinced that's a mistake.

I am a dedicated campagnolo user and have tried out di2 on one of my old carbon bikes and liked it a lot. There was a sram etap group on the classifieds for just over a grand which is imho a superior group and right around what you'd pay for a new ultegra di2.

New? Used? Wet brakes? With factory wheels? Handbuilts?

Lotsa variables. Electronic and wet brakes..I think ultegra is a great choice.

This will give you an idea of complete drive train and braking cost. I am sure you can find it for less but it will give you a starting point.
Not included in that are: seat, seat post, stem, bars, headset, bar tape, wheels, tires.

I have been using the Ultegra Di2 hydro disc for about 5k miles. It is fantastic!

yeah, not cheap, but pretty awesome if that's your bag