Plex is a great way to stream your media library to a bunch of devices, but you need to maintain a separate server for it to work — either by keeping a PC running, or springing for expensive and complicated NAS drives. Nvidia's Shield Android TV is about to solve that problem by updating its bundled Plex app with full media server capability, however, meaning you'll be able to store and watch everything all on the same box.
The Shield only comes in 16GB and 500GB varieties, so you'll be able to expand that capacity with external hard drives and use network sharing to drag content over from Windows or OS X. Once your library is stored on the Shield, in theory you'll have a fast, efficient, always-on setup that you can use to stream media to any other device.
The Plex Media Server update is included in Nvidia's Shield Experience Upgrade 3.2, which doesn't have a date yet but is said to be coming soon.