Since they had a P4 previously, anything with a Core 2 Duo will be perfect
and dirt cheap.
I would consider a Thinkpad x200 with coreboot / libreboot (works fine
plugged to my 1920x1200 monitor either via VGA or displayport through
ultrabase)
You can buy a x200 for dirt cheap but you will need some tools to flash it.
Or a G41M-ES2L motherboard which can be flashed to Libreboot internally.
Assuming they already have PSU, case etc.. that would cost around 50€ with
2x2GB and a decent E8400 Core 2 Duo.
Works perfectly with Trisquel, Debian etc.. That's what I use.
