Whatever device you have the server on, that will be doing the transcoding, not the TV itself. So you might need something with a decent processor.
I haven’t got a TV with Plex on, but as long as it’s all linked to the network, the TV should display your whole library, but depending on what the TV can play and what sort of audio device is attached, it may have to transcode.
What sort of NAS were you after, and at what cost?