OK;Ok will do.
I am going to have a go myself.
Take your time. Maybe start a new thread if you get in a jam.
Might be a good idea to build it outside the case on a table top to confirm all the parts are in working order before assembling inside the case.
Windows; you might just be buying a Product Key. You'll have to go to Microsoft.com and use their "Media Creation Tool" to build an installer using your own USB flash drive of at least 8 GB capacity. You would then boot the PC from that flash drive to install Windows. That can be tricky, so be prepared to run into issues.