I'm pretty sure both Windows 7 and 8 get the free boost to Windows 10, so that might be a solution - get an old copy of Windows 7 cheaply and then just upgrade for free to 10. Cuts out the OS cost so building one's own rig becomes cheaper from a hardware perspective, plus the benefits of customisation.

Rax, one thing to look out for is just ensuring all your parts work together. Nothing worse than picking up a shiny new GPU just to find out your motherboard has the wrong chipset or some shit.