- Ubuntu Server (with or without GUI, comes with just command line, but you can install a GUI)
- Debian
- Ubuntu Desktop
- Mint
- Pretty much anything Linux-based should do well.
- Windows Server, you can use a trial.
- Don't go older than 2012R2, IMHO not worth it.
- FreeNAS
- Standard Windows OS
- If you go Win 7 for a server OS, there's a couple of reg edits you can make to help with file sharing and network session issues
If it were me I'd probably go with Ubuntu Server and run it headless. You can connect via Putty, setup Plex and storage, then the rest of it is done via WebGUI. That being said my server runs on Windows Server 2016 as a Hyper-V host. My Plex server is hosted on a virtual machine that is hosted on my physical server, works great and has for years now. The nice thing is if I need to migrate hardware, I migrate to another Windows Server Hyper-V host via live migration or replication w/failover, and I'm good to service my hardware. You can do similar with other virtualization options...but not sure your hardware supports it.
I also run Plex as a service, so that when my file server (also does print server, teamspeak3, and network monitoring) reboots for updates, Plex starts up without me having to login. That's also worth knowing about if you go the Windows route:
To keep things simple, you might even consider the desktop Ubuntu so you have a built-in GUI to use, or go with Mint or something lighter weight. I've never been a big fan of FreeNAS, it works well, but for Plex I prefer hosting it on a straight Linux or Windows OS rather than FreeNAS. Personal preference on that aspect though.
Not sure if you've read through this link or not, but it's worth looking at:
https://support.plex.tv/articles/200288586-installation/
Frankly with the age of that system, you might not be happy with there results if you have to do a lot of encoding. I'm not sure what the Q8200 is capable of performance-wise, it should do okay for a couple streams or so I'd imagine, but it will struggle too. I'd also suggest a smaller, more efficient GPU as you won't be using that GTX2xx to help encode (I could be wrong here, but last I checked it was newer GPU's iirc...).
This article about CPU performance is worth a read:
https://support.plex.tv/articles/201774043-what-kind-of-cpu-do-i-need-for-my-server/
I've been on Plex Pass Lifetime since May 2015, and have been very happy with my decision. Had one issue a couple months ago that Plex support resolved fairly quickly and beyond that between my mom and sis in AZ, my other sis in WA, my family (wife and 2 kids) and myself streaming from it regularly, it all works great, stable and no complaints! I am eagerly awaiting Charter's synchronous cable to hit our area as 10Mbps upload is choking us down some, but overall the fact I can run multiple streams of music, video, etc. has been just awesome.