There is a free preview of 8.1 available to download and try right now but the retail version will of course cost actual money.
I would recommend using W7 to build the PC and install the preview to see what you think of W8.1
There is a learning curve (quite a steep one in switching to W8 as it does a lot of things differently to W7. By which I mean the UI (User interface is different)
As an example I am using it now and then and trying to transition yet simple things like opening and closing an Email are overly complicated and not readily obvious.
Mactronix