Let's be honest here, your grandma won't buy anything without help from someone. The leap from the iMac to a monitor+mini is not very far, with the help that will certainly be given.
Having been that "help" for many years for parents and grandparents, I can guarantee you that having a setup with as few components and cables as possible that break is more for my sanity than for their comfort. As folks get a little bit older and less nimble, random cables get pulled and drinks get spilled on keyboards.
You can book an appointment at the Genius Bar and they'll run you through how to set it up. I'm sure even the proverbial grandmother would feel confident enough to set up an iMac after that. Maybe not a Mini, though.