I'm about to repurpose my desktop machine, which has also been pulling dual-duty as a file/media/plex server, to a dedicated server using either FreeNAS or UnRAID. The machine is completely over-specced for my daily use and is only comes close to its full potential when transcoding multiple plex streams simultaneously. My i3 laptop or even an Atom X5 powered tablet is powerful enough to handle my day-to-day web/email/office productivity computing tasks.

I currently have a 2TB media drive that's about 2/3 full, so I'll add more 2 more 2TB drives to create an array, but I have to decide if I want to add ALL the drives (4 new, 5 total, 8TB storage with single parity) now and use FreeNAS (ZFS/RaidZ doesn't support adding single drives after the fact) or pay for UnRAID now and have the ability to easily add single drives as needed. The other upside of UnRAID is it seems to have an easier, more matured, and more supported implementation of Plex as well as the ability to more easily run VMs.

I haven't made a decision yet, but UnRAID seems like it will be the easier approach on a few different levels but require paying for software which is always a hard pill to swallow when there are free options available.