Before choosing your perfect Plex NAS you need to be sure what sort of media you will be streaming. Are those mainly HD and Full HD video files or maybe even 4K. Also what file size, bitrate, framerate and compression format as well as color depth mode your videos mainly have.
If you plan to stream videos only in your local network- all you need is a NAS with Plex compatible processor. To experience smooth streaming you also need to make sure that connection between your NAS and a TV or mobile device is fast enough to stream the chosen video format. This mainly applies to 4K media only.
Streaming over WiFi
WiFi will be definitely fast enough to stream up to 1080p (Full HD) videos. For something more difficult like 4K movies you either need a 5GHz router at home or at least wired network(or NAS with 4K transcoding engine). Good streaming only NAS models are Synology J series or Qnap P/P2 series.
Streaming in a wired network
Streaming over HDMI
If your wifi , lan