With those braze-ons an IGH build would be pretty lame, in my opinion. Have you ever built an IGH hub up? Good lord, man. I know you save some weight by not having derailleurs and whatnot, but IGHs with more than 2-3 ratios are ungodly heavy.

I wouldn't fender it. I think I would change that headset for a silver one and then measure my brake reach with a 700c wheel, then see if it would fit these. I would also measure and see if 650b x 42 would fit. Then, I'd probably Shimergo it. Silver Veloce 10s brifters, Shimano 8s silver long cage RD and a nice compact double crankset. Unless the top tube on that bike is much shorter than your other bikes I wouldn't bother with mustache bars. If you've tried them and didn't like them, I doubt this is the frame that'll change your mind.

Cool frame, man!