Guys, don't waste your money on a Mac to run a Plex Server. I do that as a VM on a Sandy Bride i7 PC running ESXi, it does 4x 1080p transcodes while still running a few smaller VMs. I'm about to add a NVidia P2000 for GPU transcode.
In fact, you can buy pretty much any cheap used PC w/ a PCIE 3.0 slot, slap in the $400 GPU and be doing multiple 4K transcodes without breaking a sweat. This whole setup will cost you less than even the base price of a Mac Mini. You would have plenty of cash left over for a NVidia Shield as the Plex client or more gigantic hard drives for media storage.
Even if you don't care about 4K or transcode performance, go buy a NUC. Don't pay the apple tax to just run a Plex server, you would get way better bang for the buck using PC hardware.