If you go for Windows 8.1, don't get the OEM. Get the full version. The latter is only $10 more on Amazon($107), but the licensing terms allow you to freely move the license between machines. OEM copies are tied to a single system per the license agreement.
Here it is cheaper! Hope I'm not too late! Just got this for my build coming up as well.
For what you want to do, you'll want to get the Full edition. (From all standpoints.. licensing, cost, and being able to transfer the license to another system later on). Amazon has it the cheapest. The Full edition also comes with both x86 and x64 media. (If you have a netbook with no ODD, you can use the online installer also linked below)
http://www.amazon.com/Microsoft-Windows-8-1-Full-Version/dp/B00EDSI7QO
http://windows.microsoft.com/en-us/windows-8/upgrade-product-key-only
Don't go to /r/softwareswap or any other place where you're buying keys for like $10-$30. Those are almost always being sold illegally against licensing terms. (People pawning off spare MSDN/Technet/Dreamspark keys, corporate techs writing down keys off machines' COAs, and many others sources) Sure they'll probably work, but there is no possible way you're obtaining an end-user license through legitimate channels at those prices.
el_seano's tip about Dreamspark is good if you happen to be a student or uni employee, but not entirely correct. Eligibility for Dreamspark and MSDN depends on your university, your specific school within the university, your status as a student/faculty, and so on. Having a ".edu" email address guarantees nothing.
Any reason you don't want windows 8.1? It's $105 from amazon and it's a sealed copy