Buy a Raspberry Pi and take a look at your local recycling center. You can find some quite decent monitors (even HDMI), keyboards, and mice in their electronic scrap bin for free.
I think the one thing RE: used laptops that always has to be accounted for is it requires a fair bit of technical knowledge and some learned experience of what a good used device should have.
I'm related to a few teachers and used laptops were always an issue because they couldn't afford to take the punt of getting used ones so they had to pay a premium from established resellers instead at which point you're approaching budget new laptop price territory.
Pi 400 really should have some obvious clear low cost screen upgrade option though. Possibly even just an app that it could stream to out of the box? Obviously wouldn't be perfect but if you assume there's a not unreasaonable chance they already have some Android device it could suffice.
