Help with Windows key shortcut
Help with Windows key shortcut
You're getting your first PC soon—it's going to be independent. The Windows 8.1 you'll receive from g2a is the official setup, so using their provided code is correct. You don't need to worry about formatting your hard drive unless you want to; the installation process will handle that. If you ever lose access to the OS, you can reinstall it from a fresh source.
The OS is exclusively tied to the motherboard. Only the OEM edition can be installed initially. Other retail versions can be applied to different devices, but not simultaneously.
Sure, just make sure you obtain an OEM key. That way you can avoid the issues with G2A.