Yes, the Windows 8.1 ISO can be placed on a USB 3.0 flash drive.
Yes, the Windows 8.1 ISO can be placed on a USB 3.0 flash drive.
I have installed Windows 8.1 using USB 3.0 several times with no issues, it's not a problem.
You purchase it from Newegg and receive a download version with an ISO link and key. You then use a tool to mount it on your USB drive, making it bootable for the new system.