1. I recently did a build for a friend using the Sapphire RX 580 Pulse and it was extremely easy.
2. I did not have any problems installing High Sierra using the method I detailed in my build description. I have not done a clean install of Windows in a very long time but all of your components are "PC" components and I don't see Windows being a problem.
3. I have never used the Noctual fans that you've chosen. I just used the standard brown/beige ones. I don't know if the ones you've chosen will be as quiet as the ones I'm using. I suggest searching around the internet to read about others' experiences with these.
4. I think 750W should be fine. The build I recently did for a friend just has a 650W power supply and it's been fine.
5. High Sierra has native support for NVMe SSDs. That being said, not all brands of NVMe behave the same on macOS. I've only used the Samsung 960 EVO and it worked well.
6. If price is the same, may as well go for the faster one. Lol... Remember to check the Asus "qualified vendor list" for RAM compatibility.
Here is the Z270 QVL. I was unable to find the Z370 list...
7. I would suggest going with the BCM94360CS2 w/ M.2 adapter. I use a Magic Mouse 2 and Magic Trackpad 1 with my hack regularly and all features work fine.
