Plex will require a computer running, it will take stuff from your NAS transcode it to the format you require and stream it to your device, what ever it may be.
I just recently set up my server with a 2012 Mac Mini, and I am truly surprised how well everything just works. Stoked for the release of their app.
Can I have VideoTS file structure and Plex able to read it and play on ATV? I have tons of DVDs and the last thing I want to do is to make it all as Quicktime. Also I like the menu and file structure of DVDs, so I don't want to lose it.