The main concern with old hardware is probably powerdraw/efficiency, depending on how old your PC is, it might not be the best choice. But remember: companies are getting rid of old hardware fairly quickly, they can be a good choice and might be available for dirt cheap or even free.
I recently replaced my old Synology NAS from 2011 with an old Dell Optiplex 3050 workstation that companies threw away.
The system draws almost twice the power (25W) compared to my old synology NAS (which only drew 13W, both with 2 spinning drives), but increase in processing power and flexibility using TrueNAS is very noticable, it allowed me to also replace an old raspberry pi (6W) that only ran pihole.
So overall, my new home-server is close in power draw to the two devices it replaced, but with an immense increase in performance.
The main concern with old hardware is probably powerdraw/efficiency, depending on how old your PC is, it might not be the best choice. But remember: companies are getting rid of old hardware fairly quickly, they can be a good choice and might be available for dirt cheap or even free.
I recently replaced my old Synology NAS from 2011 with an old Dell Optiplex 3050 workstation that companies threw away. The system draws almost twice the power (25W) compared to my old synology NAS (which only drew 13W, both with 2 spinning drives), but increase in processing power and flexibility using TrueNAS is very noticable, it allowed me to also replace an old raspberry pi (6W) that only ran pihole.
So overall, my new home-server is close in power draw to the two devices it replaced, but with an immense increase in performance.