If you have a Mac Mini already, install your Plex server on this and then any storage option will do. I have a cheap Zyxel NAS linked to mine. The MM is powerful enough to transcode to your other devices (if necessary), runs Plex HT for playback itself very well (especially when coupled with an Apple remote) and, crucially, goes to sleep when not being used and wakes up on request (even remotely - if I open the Plex app on my phone when out and about it wakes up the MM to display the library).
A NAS that will transcode is the neatest solution by far, but use the MM if you already have one.