There are a few problems with your specific request, and I would guess it's because it's difficult to find the documentation surrounding the questions you are asking. The short answer is you can't do it.
First, the C2100 is based on the now 10-year old Westmere-EP platform. For FreeNAS or virtualization, this hardware is still very relevant for home use, albiet it uses alot of power. However, there are a couple of things here that are important. Intel does
NOT typically have integrated graphics in it's EP line of dual-socket servers, and they definitely did not during the Westmere era. Integrated graphics are the component in hardware that allows Quick sync to work. So any CPU parts you find in the list of quick sync support processors in the link you posted would be incompatible with your motherboard.
In addition, not all Quick Sync is created equally. The original Quick Sync enabled processors, form the Sandy Bridge architecture (2 generations newer than your CPUs), do not support the same codecs that the newer CPUs do. So even if you had something from that generation, it wouldn't be able to do H.265 hardware acceleration in Plex.
You may be asking next, well perhaps I will just put in a GPU to solve the problem. Unfortunately that wouldn't work either. For reasons having to do with jails, hardware passthru and virtualization, you can't do it in FreeNAS. You would need a hypervisor capable of giving the guest operating system direct access to the GPU. Even if that was something supported in FreeNAS, you wouldn't be able to do it in FreeBSD anyway. Since the plugin is running in FreeBSD, and Plex doesn't offer Hardware Acceleration in FreeBSD, it wouldn't ever work.
For the record, I'm not even sure why you are trying to transcode a 4k file. Please refer to Plex's own community forums to learn more about that topic specifically.
NOTE: for 2022 – Plex has come a long way since this FAQ was originally written, HW transcoding has become more available and more stable, and tone mapping was recently added to address the hdr/sdr color conversion issues. The first 4 ‘rules’ generally are no longer as important as they once...
forums.plex.tv
But for all of these reasons, and others, are why I have a separate VMWare server running. FreeNAS is for storage, VMWare is compute. Sure FreeNAS can do some compute in a pinch, but it's simply not it's mission. With TrueNAS SCALE coming next year, that may change. For now, you might be better suited looking at Proxmox.
