The hardware is totally fine, Linux requirements didnt really change at all in the last years.
KDE Plasma is a really well maintained desktop, poorly also with a ton of customization. It has a very familiar user experience. GNOME is also nice but not familiar at all.
On these machines, recommendations:
some stable distro like Debian 12, with automatic background updates
OR an atomic distro like Fedora Atomic. (Still waiting for CentOS bootc, which would be the best of both worlds. Or Rocky/Almalinux Atomic)
GNOME or KDE
best would be to always delete the user account, so they need to store stuff on a network drive. That way they cannot permanently break a desktop, but you still dont need active directory stuff.
Be aware that managing many PCs is work. Keep it as simple as possible, install apps as systemwide flatpaks, keep the OS minimal, automate updates.
Maybe have a look at ansible, I think it is complex but the learning curve is worth the effort if you need to manage more than 4 machines.