Although power is free for you for some reason, buying a good PSU gold rated and above is important, you don't wanna fry or burn your S5 miner in less then a month.
Seasonic 760watt PSU is a good choice, you can choose either platinum grade or gold grade.
Ok I will try to go for at least silver+
Also is it good to buy a bigger PSU and then run 2 miners on it?
Edit: e.g. http://www.amazon.com/dp/B00COIZTZM/?tag=pcpapi-20 (1300W) OR http://www.amazon.com/dp/B00K85X2AW/?tag=pcpapi-20 (650W)
You can buy 1300watt and run 2 miners with it, but I prefer 1 PSU per miner as it is cheaper and usually the psu last longer.
Note: Also I dont know if anybody have this experience but warranty for smaller PSU tends to be much faster as they have lots of stocks. And for big psu like 1300 watt and above, sometimes I have to wait like 2 months!!!
Thanks for the heads up. Im going with 10 of these: http://www.amazon.com/gp/product/B00K85X2AW/ref=ox_sc_act_title_2?ie=UTF8&psc=1&smid=A1CPWE4BUHBCWE
(EVGA SuperNOVA 650G1 650W ATX12V)