Just don't go with used server PSUs that don't have proper protection, don't try to save 50 bucks on a $2K+ rig.
I use 850W power supplies for my 5xRX570 rigs, so even 1000W would sufficient for you if you plan to undervolt your cards.
My rigs take 500-550W from the wall on cryptonight, 600-650W on ethereum mining and up to 750W on dual mining or some gpu-intensive algos, like neoscrypt or lyra.