Do I need to buy a full, DVD copy of Windows to run Windows?

Yes, Microsoft do not give Windows away for free.

If you want to, but you can get Windows for less than that.

Amazon is the vendor in the picture I posted, there are others.

Click on the picture, it's a link.

No.

But Windows is Windows, buy from whoever is cheapest (just don't buy Windows Starter Edition)

You're welcome.