For the OS just go with what ever vesion of windows you want (not going to explain it in detail but 7 is generally regarded as the best, 8.1 is decent with classic shell, and 10 is some shady spyware stuff but it gets updated frequently if you have question just look through a few threads on choosing your OS)
Definitely go with a used GPU you'lll crush any price to performance you'll get with a new card. I'd suggest a 970 there's a few floating around from 100-200$ if thats oustide of your price range look at the NVIDIA 700 series or some of the AMD r9 cards.