if you are going down the route of putting all your stuff on a NAS, so that your computer can be off, then be aware that this won’t work with an airport express.
You can host your music on an ‘itunes server’ on a NAS, but this is little more than a shared repository; you still need to have a computer on in order to play music out of an airport express.
Have a look at the various logitech squeezebox devices. They can stream internet radio (including bbc stations) straight from the web, or your music from your NAS with no computers turned on.
I’ve got a netgear readyNAS and a squeezebox touch (plugged into my living room amp); I’ve also got a sony bluray player which can stream videos by DLNA from my NAS. Very impressed with both, and I think I’m going to get a squeezebox radio for the kitchen, and maybe another for our bedroom.
Dave