Plex server side realtime transcoding isn't really working on Bpi-M3. More CPU capacity is needed (or active cooling for that, but the silent operation is gone with a ventillator). Another issue is that plex server DLNA component has memory leak problems sometimes (that's why I scheduled daily reboot for the system).
Apart from these ( ) I'm satisfied with my current Bpi-M3 config. Ca. 5 plex clients are playing simultaneously SD media from the server. Full HD media max 1-2. Transmission, file sharing, SCP/SFTP are working good. The notorious SATA interface was never the bottleneck for my usecases, I simple don't need more speed than is available. My HDD get power from an external adapter, so it won't reduce the amperage left for the bsp itself.
No clue about M2U, reading the specs, it's all the same. I think, you don't need more than 1GB memory for your NAS, and if the realtime transcoding isn't working, you don't need 8 cores. Playing 1 SD media (without transcoding), 350 MB memory and 1 cpu core avg 20% peak 80% is needed for me on 16.04 ubuntu mate os. And I personally would not need SATA interface, the speed of usb2 and an external SataToUsb converter would be enough.
Plex unofficial ARM repo: https://www.dev2day.de/typo3/projects/plex-media-server/