My Mac sits headless hard wired to my asus Ac68r router and local clients in the house consists of 3 ROKU 3’s and and Apple TV 4 which in my opinion has the best looking Plex interface. My internet speeds are 150 Mbps download and 10 Mbps upload. All of my movies are in Mkv x264 1080p and shows range from 480 to 720 mkv x264 and everything seems to play just fine. Internet speeds and bandwidth will play a big role in your satisfaction especially if all your media is of higher bitrates. Besides plex the only other apps I run on Mini is iTunes and photos so I can integrate it with my plex library. The Mac should be able to handle all of your transcoding and server needs. It is able to handle up to 6 streams of mixed iPhone android and other clients such as rokus and Apple TVS. As far as the mini being my server I have a plex pass and about 15 members of my family and friends in my Plex Home. Only issue I’m running into is running out of ports and storage space. It’s on current version of Mac OS sierra and current version of PMS 1.3.0 and my setup consists of 12tb of WD mybook externals. I’ve been running my plex media server off my 2010 Mac mini server intel core 2 duo and have had no issues for the last 4 years.

Mac mini plex media server transcoding