This totally depends on what they need and what they are willing to spend. If they just need access to google docs/ sheets etc for example (so just the web with a web browser), either Linux / Un*x or Windows will do and linux is the free choice. If they want to be part of the microsoft ecosystem or use some windows specific software they will need to use Windows or an emulator in Linux.
I would not recommend MSDOS in this day and age.
The answer is the microprocessor
Bending, twisting, or the overall age of the cable, or other stuff like that.
You should know the units :))))))))))))