The farther you advance beyond a disc player plugged directly into your TV, the more complicated it is going to be. To simplify it for the non-techies in your household, you are going to have to invest a substantial amount of your time, varying from programming a programmable universal remote to assembling and configuring computerized systems.
The few NAS units that I've investigated are only that: Network Attached Storage. They use USB only to expand their storage capacity, not to connect directly to a computer. If you want to use them for audio/video storage without an intermediate media server (e.g. an HTPC or RasberryPi running XBMC), they usually need to support DLNA, too. Most don't, but very few audio/video devices can use either of the two other dominant file sharing protocols, CIFS (aka SMB) or NFS.
Usually the easiest way to start at providing movies-and-music-on-demand
is a hard drive connected directly to a Blu-ray player. You still have the annoyance of making sure both that the disk has been formatted in a way the BD player recognizes and that the files you put on it are in formats that the BD player recognizes.
Some of these topics are probably better discussed in the Home Theater Computer forum at http://www.avsforum.com/f/26/home-theater-computers