You can download from Microsoft at any time, even choose whatever edition, like 7,8,10,pro etc you wish. After installing. Windows will have a trial period where it wants you to register the key. That's when you'll need to purchase a key for whatever edition you have, and it'll be tied to your Microsoft account with the specs of that mobo. You won't receive a hard copy of windows unless you specifically choose that or buy from a vendor like Walmart, Amazon, newegg etc which had both a small flash drive and included key, so once you get online to Microsoft, it registers your copy for you.
It's one of those things where there really is no rush.