Gday.from your first link, please note, Windows 8 EULA
"What about upgrading the software?
The software covered by this agreement is an upgrade to your existing operating system (XP SP3, Vista, or Windows 7) software, so the upgrade replaces the original software that you are upgrading."
From your second link,
"According to Microsoft, the PC you want to install the Windows 8 upgrade on needs traces of an old operating system that is eligible for the upgrade. While you can still install Windows 8 on the system, it won't activate otherwise as no previous version of Windows has been found by the installer running on it."
Also note, you have to hack the installation to get it to activate. Microsoft soon picks up on such practises and they get blocked.
You do not have a legal copy of a OS you can first install on your new build, and then upgrade.
I would, buy from a reputable dealer, (see link) Windows 7 Full Retail. You could if you wish move it to a new build in the future and, upgrade to Windows 8.1 if you really felt you needed it