If you're going to hand out Ubuntu CDs, make sure it's a current LTS version.
It doesn't need recent hardware to run, though I have doubts about physically old hard drives. If the machine can be fitted with a current-production SATA drive there's not likely to be anything dreadfully out of date. Fans also wear out, but can be replaced.
If you're making a donation. a new 500GB hard drive makes it not-junk, and also covers you against data security issues. Pop Linux on it, and you're also covered on licensing issues.
That bunch of machines in New Ireland, I wonder if they have on-board graphics hardware because I'd expect it to be VGA. Performance would be OK. Change the BIOS setting (some boards can autodetect) before you remove a graphics card.
Some people can likely think of ways to do these things better, but I reckon these are the things you need to get right for a worthwhile donation of old hardware.