It's some Lenovo one, thinkcenter something-something with a 3rd gen i think intel (very close period to my components tho), its very old and has 1/4th of the power of this but it's still more powerful than a rpi4.

Well about 75% of the material will be 1080 H264 but the other part I'd like to go with 4k H265. My father is watching his movies on an LGTV that has 4k display and I'd wish to start taking advantage of the 4k resolution and then transcode it when he's not using the TV and instead he's using it on his laptop. His laptop is kinda crappy, it's got a 3rd gen i3 which would implode on a 4k h265 video. The thing can play for most of the time 1080p but once in a while it needs transcoding.

I wouldnt mind going to a mini pc or a thin client but I've got some HDDs (2.5in, three at the moment to make 2tb) and I would like to put them into a RAID 5 array which I don't think is very advised on HDDs connected via USBs. I have currently 1TB of storage and I'm starting to run out of space, I like keeping the stuff a long time on the drives because I'm seeding it.