So if you plan to use it long term I would search for a lenovo/HP/dell mini pc, it will be worth more in the long run, if i's just a stop gap the mini will be ok, but not optimized at all for the task, when (and if) plex updates to support native transcoding on M1 then the story changes and the M1 becomes a AMAZING server.

the m70q is cheaper to fix if something breaks and can be upgraded just as easily if the need arises (can go up to 64gb of ram and a i9, also it has both a nvme slot and a sata.

Mini M1 has 2 USBA gen 3.2x2 10gbs and 2 Thunderbolt 40gb/s, m70q has 4 USBA gen 3.2 5gbs and 2 USBA gen 3.2x2 10gbs You have more ports to connect drives and docks for cheaper unless you have a setup to take advantage of the thunderbolt ports which is pretty overkill.

Plex Great benefit from Intel CPUs as it can use Vsync to transcode without huge performance penalties, Plex server currently is not optmized for the M1 and so even being drastically more powerfull than the i3 on the lenovo performance will be on par or worse in general in transcoding tasks.I have a cheap dedicated plex machine at home, a Lenovo m70q with 8gb of ram and a I310100T, same size as the mini, consumes 10-15W on average, I would recomend something like this over the mini for the following reasons:

Plex media server mac mini requirements