I was thinking about that same thing. This is my second build and is actually for my cousin. I have a product key for Windows 10 from my first build and was wondering if that would work for this build. If it does work, then I could budget in the water cooler and ssd!
Depends, if the Product Key is already used to register a functional Windows, then NO, but if it's not being used or the windows was uninstalled, then YES.
If you don't have any, I can help! I've bought a lot of windows DVDs over the years, the disks usually get scratched out or break, so I have alot of Product Keys to spare, which I just give away to my friends or other family members if they need any. So I'll PM you a spare Win 7 key I've got lying around later(Because Windows is alot cheaper in my country ).