It depends on what you are running, but at one point I had an Odroid N2+ with 8GB RAM running Home Assistant, mpd, Snap server, zwavejs, mympd, jellyfin, and Calibre, all in containers, controlling the house and providing music for the sound system, playing movies, and with no issues. It ran for 7 years. So you don't need much; memory helps.
Oh - I take it back; after I put Jellyfin on it, it would struggle with transcoding. No GPU, old, weak CPU, whatever. But otherwise, it was fine.
At some point I realized I'd have to leave the computer with the house, because I have over 30 hardwired z-wave devices I'm not taking out if we sell, so I moved all of the services except Home Assistant and zwavejs to another computer.
My point is: old computers should be fine, assuming you're not trying to run LLMs on them. Or going heavy video transcoding. Just for serving up some web applications? You don't need much.