I like lenovo thinkcentres I have a 700 series and a 900 series. At the time, one was less than $100 and one was $150. I put another ~$80 for old ram and old SAS drives. They’re working great and I have what I consider to be huge storage. Things are different now, but used office computers are always going to be plentiful, the only limitation is how old you want to go. Good news is that Debian will practically run on anything and doing docker via SSH CLI isn’t hard, especially if you get Dockhand or Portainer(for now) running first. If you’re serving data, 256gb won’t get though.