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.
If you want to pay less for seasonic you can also take XFX. These are actually rebranded Seasonics and are also cheaper.