The Seasonic S12II-520 should be enough even with an i7-7700K + RX 480 and basic components connected to your motherboard/PSU.
If you want more headroom for almost the same price, the S12II-620 is only $2 more expensive:SeaSonic S12II 520W 80+ Bronze Certified ATX Power Supply
($47.89 @ B&H) SeaSonic S12II 620W 80+ Bronze Certified ATX Power Supply
($49.90 @ B&H)
But my personal choice would be the Seasonic G-550 as it is now cheaper than the modular version of the S12II (i.e., the M12II), if you can add ~$20 to your budget.SeaSonic G 550W 80+ Gold Certified Semi-Modular ATX Power Supply
($67.99 @ NCIX US) SeaSonic 520W 80+ Bronze Certified Fully-Modular ATX Power Supply
($69.99 @ SuperBiiz)