Also if i want to make a plex server on an old PC, what would people recommend?

Any desktop PC built in the last 10 years (edit: that was at least mid-range when it was built) should be fine. Just stick some hard drives in it :)

Intel processors are a good choice because their onboard GPU is quite good for video encoding/decoding. 6th gen or newer Intel Core processors (2015 or newer) would work well. They improved the H265 encoding/decoding a lot in 8th gen (2018) so that'd be even better. You can use something older but you'd need to also use a graphics card for video encoding/decoding, and it'd use more power.

Having said that, keep in mind that performance per watt almost always improves over time, meaning newer processors are more powerful even if they use the same power as the previous generation. A newer i3 will perform better than a very old i7. Using an very old, power-hungry system may end up more expensive in the long run compared to a newer mini PC.

I like using Proxmox. It lets you run multiple virtual machines on the system. VMs are good because you can easily snapshot them and revert back to an old snapshot in case of issues, and you can easily move the VM to a different system in the future. I use Unraid at home and really like it. It's a bit simpler than Proxmox, but it costs money to use (Proxmox is free for personal use).