You may think you're doing it on the cheap, building a whole rig... But then you buy the OS.
I actually already have an OS that was installed on a Virtual Machine last year, and my parents paid for that, so I'm good.
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.
I've got Windows 8.1 :D
Making sure all the parts work was my biggest worry, although I think I'll be able to use the old motherboard after all. It checks out with the processor I want but I'll have to do a little more research to make sure I get the right graphics card. Luckily it's not that old. I bought a new case and power supply today so that should arrive next week. All I need now is the new CPU. The graphics card isn't as urgent because there's a built-in one already. The hard drive is good too, I don't think I'll need to replace it.