That being said there is a recent change of a lot going with server PSU's. With these you can get a quality PSU (in a lot of cases) cheaper then a ATX power option. If you have a 220/240 I would seriously look into this option.
I believe the domestic supply in my location is 220/50hz so I will look in to these server psu's you mention.
It will only draw what's required. Most high quality ATX supplies will run quieter when not fully loaded.
When you say they only draw what's required, does that mean an electricity bill for an S5 would be the same across both a 750W and a 1500W PSU? Ie. does a larger PSU increase electricity costs?