Are you planning on running Plex off the NAS or just using NAS as storage for your Plex server? If the first thing, make sure you get one that can transcode well enough to support your use cases, else you will be sad.
first off understand your use case and then your budget. Network attached storage is not meant to be as portable as a usb drive. if you are looking for space i would recommend getting a 4 bay with a robust os/apps some thoughts here https://www.pcmag.com/article2/0,2817,2401086,00.asp