I missed that, I don't have much experience with that, but I believe you would be encoding and using the other computer to handle the stream duties so your main computer can just game. Depending on how much the i7 9700k costs, you could go for that, but honestly I would probably get the 8700k, has two less cores, and but is hyperthreaded. Also should be more widely available for more reasonable costs.
Asus Strix is pretty much the highest end for almost every component, there's a reason every youtuber uses Strix products for their "Ultimate Gaming Computer." There are tiers so you could go for the Maximus, but that's pretty overkill IMO.I would go with possibly the ASRock Taichi as an alternative though