Switch-mode PSUs are very inefficient at the low end of their duty cycle.
A 250W 80-bronze PSU for a 60W load will be operating at 25% capacity and 82% efficiency or better.
A 500W 80-titanium PSU at 60W will be at around 12% and 90% efficiency or better.
So, an 8% difference in minimum required efficiency...for a huge increase in cost.
It's much better to buy a high "tier" PSU (for reliability and safety), sized so that it spends most of its time at or above 20% duty cycle (which in OP's case would indeed be 250W.)
80-gold is very common in the marketplace and where most people should probably be buying.
For building NAS, there's a myth that you need good enough PSU to spin up all disks at once (unless you have delay mechanism). I wonder is it still matter now.
A 250W 80-bronze PSU for a 60W load will be operating at 25% capacity and 82% efficiency or better.
A 500W 80-titanium PSU at 60W will be at around 12% and 90% efficiency or better.
So, an 8% difference in minimum required efficiency...for a huge increase in cost.
It's much better to buy a high "tier" PSU (for reliability and safety), sized so that it spends most of its time at or above 20% duty cycle (which in OP's case would indeed be 250W.)
80-gold is very common in the marketplace and where most people should probably be buying.