First Build, can someone point me in the right direction please?

Okay, so a bit of backstory first:

I use my personal hardware for work with the exception of a MX300 256 GB SSD that I'll leave at my company when it's time to bring my rig home.

My specs:

• Intel core i7 8700k
• iGPU (UHD 630)
• 16 GB Trident Z RGB 3000MHz
• ASUS STRIX Z370-i
• 2 1080p screens
• integrated wifi/bluetooth module (planning to use Ethernet only)
• Crucial MX300 256 GB SATA (Company drive with windows 10)
• Intel 660P 512 GB NVMe (Blank)
• Intel 660P 1 TB NVMe (Blank)

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Questions//Concerns:

• What Vanilla Type should I go with?
• I would really like to have iMessage working
• I have a pre existing windows 10 installation on a 256 GB crucial MX300 sata that I woulb prefer to keep and be able to dual boot to is that possible??
• Ideally (if I can use my pre existing windows 10 installation) I'd like to boot windows from the Crucial MX300 drive, & OSX on half of the 1 TB 660P with another clean windows 10 install on the other half. (if that's not possible I'll just use the whole 1TB for OSX).
• If I have to do a fresh windows 10 build I'd like to have the windows 10 on the 512 660P drive & OSX on half of the 1 TB 660P with another clean windows 10 install on the other half (again....if that's not possible I'll just use the whole 1TB for OSX).