I always used my retired PCs and parts but then my kids all wanted gaming rigs so spare PCs and parts do not exist in my world anymore and they tended to be too big, noisy and inefficient.
I would go for used ex-corporate desktop mini PCs from the likes of Dell, HP, Lenovo. Perhaps don't go for the smallest ones if you want to be able to get into them and add stuff. They tend to have reasonably good idle power and noise and its common to find ones supporting two nvme ssds. Intel cpu with quicksync for jellyfin video decode if you aren't adding discrete gpu - check supported codecs. Codec support varies across generations I think.
I would stay well away from laptops: bad thermals, power limits, limited expandability and SBCs like RPi which have poor io for servers.
I picked up an old HP Elitedesk off ebay a few years ago. I added a few TB of SSD and another stick of DDR4 when that stuff was cheap. It supports two nvme ssds as well as space for sata drives. Apart from media storage I can't see any compelling reason to want to upgrade it.
I would take the old ones back from for profits, put them in the hands of a non-profit and keep them at a sensible price. All this land speculation is bullshit and contributes no value to domain users - its all a get rich scheme for a small number of insiders.