If you're hell bent on relying on the computer built into a NAS rather than the one sitting on your desktop to stream music, then by all means make sure you get a good NAS if your library exceeds anything over a few hundred albums. But if leaving your desktop computer running isn't a big deal, what is it that you can do to store your music?
The two choices you have are an internal or an external hard drive. One my setup I use both: most of my music is stored on an extra internal HD and I back that music up with an external USB drive. This works fine for me because my computer is capable of having a few drives added internally - while most aren't.
Another alternative is to make sure you have a large HD in your computer and use it for music as well as programs. I am less fond of this approach because it doesn't take too long before your music storage gets into the Terabyte region (a thousand gigabytes) and that's a really big internal HD.
I think a better alternative for most of us is to put all your music on a USB HD. You can purchase a 2tB USB 2.0 HD for $100. That is enough storage for a couple of thousand lossless albums to be stored - which means approximately 20,000 tracks which is a lot of music.
Backing up this drive then becomes simple and safer than anything a NAS RAID has to offer. Simply purchase an identical HD and before you go to bed at night, set the two to copy. Then take your second HD and put it in a safe storage area. Update it every now and then.
Your cost to build a library and back it up is about $0.10 an album. Ten cents! That's five cents per album to store and play and another five cents to keep a safe copy.
Amazing what a nickel will buy these days.