cheers
Cost effective option for mining will be server PSU + breakout board.
Wherever ou buy them, they will always be more cost effective and reliable than ATX PSUs.
For a single S5, or 2 underclocked, a DPS1200 FBA off Ebay or similar places will cost 25-30€
Breakout board + 8 cables will be around 60€ shipped.
But if you already have several single rails PSUs over 450w, you can use a separate one for each panel of the S5.
I assume you don't have a second S5 available, or you could try to swap controllers.
Also, you can try to start with only one panel at a time connected for the data cable (16 or 18 pins depending on revision).
If nothing works, check if it's still under warranty and send the whole miner to the address Bitmain will give you.
If it's not under warranty, and depending on how much you'd want for it, I may be interested to buy it to salvage some components for my projects, (assuming you'll ship from UK, and I'm in France).