CPU - If you have Intel you can do this - it creates a linux USB you can run the intel processor diagnostic tool from -
http://www.tcsscreening.com/files/users/IPDT_LiveUSB/index.html
Memory - download the free version of memtest86 and create a USB, it is bootable so don't need windows. Check sticks 1 at a time, any errors are too many
Motherboard - not sure as its sort of last thing, if all the other parts work but PC won't then you have to look at motherboard but only then.
Windows - if you buying win 10, you can install it without a licence and buy one after... just grab the
Windows 10 media creation tool and use it to make a win 10 installer on USB and put it on PC
when you reach the screen asking for licence, click "I don't have a key" and win 10 will continue to install and remain unactivated until you buy it.