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?