I think either MediaMonkey or WinAmp will work just fine. But you might have trouble accessing the iTunes files until you remove the DRM and convert them to FLAC or MP3 (I recommend spending the money on AudialsOne software).
You could also try the software version of Squeezebox (softsqueeze). Although I'm not sure if your NAS box would accommodate the required server-side software. You might be able to have that reside locally, though (I don't think that slimserver and the library have to reside on the same physical machine).
I used Winamp, but I know Dean and others really like MediaMonkey.
