Plex transcoding box on the cheap - which locally available HW will fit my needs the best?

Builds [LGA1151,LGA1200] NAS Killer 6.0, Plex QSV Builds
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The G4400 will perform better with exception of H.265 content.

Why are you limited to those options you listed above? Can you not get an 8th or 9th gen from this guide?