Best Mac Mini For Plex Server Free

I'm gonna be honest upfront, I'm looking to save myself a couple hours of Google boredom.
I recently got a Roku stick for my TV, I feed it via a Plex server on my laptop. The actual media is stored on a NAS that is always on, I'm getting tired of having my laptop open while I listen to music or watch a movie.
So, what is the cheapest single board computer (SBC) that will run my plex and just hum along with little maintenance from me? Hopefully the system will be nothing but an SBC and a power supply. Requirements?
1) Cheap
2) Quiet
3) Wireless
I'm assuming it will run Linux, which I'm very comfortable with. But I'm not ruling out other OS's.
Thanks!

Best Mac Mini For Plex Server Storage

I'm using a 2012 quad core Mac Mini Server exactly for that purpose, but instead of a NAS, I have an external Thunderbolt Raid-6 enclosure by areca. Plex is the latest and best iteration of. I was using an i7 2012 Mac Mini as my plex server and it did pretty well but it cannot output 4k to a display. I just ordered the base model Mini but upgraded the storage to 256GB so I am hoping it runs a bit better than the i7 2012. I skipped the 2014 models because they were for the most part slower than my 2012.