Keep the NAS & Plex devices separate, follow the quicksync guide with an HP290 for plex, and build any NK4 guide for solid unraid nas functionality. The HP290 will provide you with the capbility of ~21 plex transcodes.
Nope.
If you want to keep these as 1 device I believe the rule of thumb is ~2k passmark/transcode. So it all depends on how many transcodes you need (And bear in mind that direct play takes basically nothing from the cpu, it’s only the transcoding we’re talking about). I went this route at first, but the quicksync box has been MUCH more reliable.