Problem is were in technology limbo
The old boxes like WDTV are no longer developed but things like the Roku which are all the rage are not quite as capable as NAS media players compared to the WDTV, especially if you have produced raw disc structure backups. You could run folders through MakeMKV which would extract the movie lossless to MKV container but the Roku would still likely have trouble playing the videos through Plex. You'd need a server with a powerful CPU that could allow the Plex media server to transcode the content in real time making it playable on the Roku.
What we are waiting on is AndroidTV which is basically GoogleTV v4, AndroidTV runs a version of Android 4.2 so it gets Amazon prime instant video but also has the means to run XBMC which will handle the media decoding from a NAS. But AndroidTV devices aren't yet available and the only one known to be coming is the Hisense Pulse Pro hopefully in the next few months. Pivos may also (not totally sure) have a AndroidTV box in the works and their version would support HD audio, it could turn out to be just a regular Android tablet box, still even if it were you'd get XBMC and possibly Amazon video via the tablet app.
Plex is also an option on the AndroidTV boxes but Plex doesn't handle disc structures so well, you'd be better using MakeMKV on the folders to extract the movie, the upside is that the Plex server would not need to transcode the content, the AndroidTV boxes should be up to the task.
There aren't really a whole lot of options for what your trying to do, platforms that support disc structures on a NAS and Amazon video are very rare. The bulk of Amazon video platforms use DLNA media players similar to Plex (without the coverart)
* Game consoles = DLNA
* BD players = DLNA
* Smart TV = DLNA
* Roku = Plex
* GoogleTV = DLNA, Plex or NAS (but not very good at that, as no XBMC)
So a PC is probably the only real option right now or wait for Amazon's own rumoured set top box (which runs their Android fork so that gets you XBMC).