Hi Stuart,
I’m assuming that you are planning to develop accessible game. I can understand that you were recommended to buy the mini, I think you need to go there at one point since it’s a full fledged accessible solution. To develop for eye tracking, you could start with the 4C. I would recommend to move to mini at some point to get a better feel for our interaction patterns etc. for accessibility.
I share a few accessible games released during last year:
https://blogs.microsoft.com/accessibility/eyes-first-game/
Please keep us updated.