I had a QNAPS NAS as a media center in 2013 but I didn't enjoy using it.
For my next build I want to recycle my Intel i5 2500k CPU and chuck some high capacity NAS drives into a new case. For the case I might pick a windowless Fractal Design R5 or R6. The NZXT Phantom Red tower might get donated or something. It's huge, bulky and a dust magnet.
The i5 2500k would be more than sufficient for plex and media server purposes. It doesn't come with ECC ram capability but that's fine.