And having a unified clippboard is just so convenient

If you go that route I’d want to make sure the cpu is at least somwewhat recent.

My first server was an old office PC I bought used as well, but I had real problems with it, because the CPU was lacking some X86 instructions which is why I could not run a specific service I wanted to as it used those. (And if you want to run jellyfin in the near future you should make sure to buuy a cpu that can also do some hardware encoding/decoding as doing that in software on a low end CPU can make the experience somewhat sluggish.