I, personally, would suggest Debian 12 especially since they still supply 32 bit ISOs. (Also 64 bit, but that's kind of a given at this point)
- Debian is rock stable due to testing like crazy
- Adding a lightweight desktop like XFCE would help with not overloading the PC
- If I remember correctly, updates for the next 5 years since it's a long term support (LTS) release
- I am guessing you mean 75 GB which should still have, at minimum (absolute worst case everything went wrong kind of wrong), 60 GB left for programs and files
Since they provide both 64 and 32 bit ISOs and run the same thing, all support issues can be done exactly the same on all the computers since, I assume, there is no dedicated graphics card (Nvidia, AMD) in any of them.
Here's a link to the downloads: https://www.debian.org/distrib/
Download from the "complete installation image" area on the left, second section down.
Edit: If you can use a DVD or USB then use the DVD link, but if they can't then the CD image will also work.