There isn’t enough information you gave to really answer all of your question. For instance…What operating system are you currently using with the i7-920? Do you have a graphics card? Are you using any ssd’s presently? Have you purchased Plex Pass? How big of a storage need do you have now or intend to have in the near future? Is the desired build only going to be for storage/Plex or are you also intending on using it for other needs?
My knee jerk recommendation is to read the link below and seriously consider getting a HP 290-p0043w (along with ssd/nvme m.2). In turn you could still use the existing hardware as a NAS and follow the guide on networking over to the quicksync machine.