Even if you do transcode a ton, the G4900 can handle it. It can do 20+ 1080p transcodes. The whole point is that a Nvidia GPU is not necessary!
If you want to run all of those services, my advice is to run them on your NAS, and keep Plex separate on the HP 290. Build something cheap like a basic [Guide] NAS Killer 4.0 - fast, quiet, power efficient, and flexible - starting at $125