OS: Not needed
Peripherals: Not needed
Budget is 600-650 USD
I am fine with using used parts.
PCPartPicker part list / Price breakdown by merchant
Depending on how much storage you require for gaming, if a single 500GB SSD is sufficient for the time being, I would take it. You can always add a second HD later when needed
Also opting for a cheaper (a bit flimsy but workable) case. Drop the fan too for now and you can squeeze in a RX 580 8GB GPU for $670
If you can afford 20 bucks over, I have this list. Balanced in price and performance, roomy case too
512gb SSD, and with room in the case if you want to throw in a couple hard drives as well.