I like the “1 liter PCs/home servers” for this kind of stuff. I have a 3 node proxmox cluster running on hp elitebook mini 800 g4’s. I got them for around $120 each on ebay (prices vary). Other big manufacturers have their own mini modules (hp, lenovo, dell) Generally these have a lower price tag than something like a similar generation intel nuc because it’s less of a niche market, these are used in business office environments and usually sold used pretty cheap when hardware is refreshed or businesses are closed. You can find replacement parts easily also. Just make sure they include a power adapter if you do search for one.
Mine are running i5-8500t processor which supports Intel quicksync and performs very well for video transcoding in plex. Should easily be able to do a couple of 4k transcodes easily. If you’re not interested in running proxmox, this would run OMV easily and have plenty of power to run lots of containers.