Version of Windows 8 required from scratch or not?

Apologies if this is such a basic question but I can't find a clear answer anywhere on the FAQ's.

I have bought and installed Parallels 8 for the first time. I have no previous versions of Windows installed, do I need to buy:

1)the full version of Windows 8?
2) download Windows 8 Preview first then buy an upgrade?
3) purchase the Windows 8 System Builder OEM DVD 64 Bit?

Thanks as Live Chat doesn't seem to be answering today either.