No, you don't have to buy OS X unless you don't have access to a Mac (friends', school, library, work, college or university). See the link above for more information.
You don't have to pay to make an Apple ID (account) but to download software from the Apple Mac App Store (MAS), you'll need to associate a credit card with your account even though you'll be only downloading free software.
If you use another Mac or Hack to access the MAS, you'll have to sign in with your account ID, otherwise you might have problems with the after install access.
So, create and log into your MAS account on your friend's Mac and download Sierra; after it's done downloading, the installer opens - just quit it. However, before downloading, make sure your friend's Mac doesn't have the Sierra installation app in the Applications folder; if so, move it temporarily to another folder. Download UniBeast and MultiBeast for Sierra. Run UniBeast to create the Installer USB thumb drive. After UniBeast is done, copy MultiBeast to the thumb drive. You're now set to install Sierra on your PC. See the tonymacx86 guide for instructions >
https://www.tonymacx86.com/threads/...ierra-on-any-supported-intel-based-pc.200564/
Since the Intel on-board graphics on the 100 series motherboards is flaky right now, I'd invest in a low end graphics card like a GTX 750 Ti or used Nvidia GTX 760 or GTX 950. Stay away from AMD graphics cards as they have sleep/wake-up problems.