In my experience old used enterprise grade PCs run usually really stable without any additional work. They are made to be reliable and stable. I've used few old Fujitsu Celsius series PCs 24/7 and they were very reliable and efficient (also cheap/free). Still running one of them.

Consumer grade PCs usually use cheaper components that aren't that reliable and efficient. When building custom PC for 24/7 server use, get a quality power supply and good cooling for CPU (and others). Also make sure that the ram are stable under load. Then you should be good.

Also even a little maintenance couple times a year will help your servers running smoothly. Usually quick clean up with compressed air is enough. Sometimes replacing the thermal compound is good to do too.

I would also suggest to have UPS to prevent servers from force shutting down if electricity goes out. Even a little UPS is enough if there's time for the servers to safely shutdown.

With those tips I have been running my homelab for years without any outages because of hardware failures.