you know, I've been thinking of the Mini for my mum too - don't know about your dad, but my mum is definitely a technophobe. The Mini is very intuitive in its use, and he will be able to just buy the books he likes on the fly. So if you are happy with the book selection at Kobo in your language, I'd go on with it with no issue.
I'd recommend getting a cover, as it makes it easier to hold the device without touching the screen accidentally (it adds a bit of "height" to the sides if you get something like
this one, which envelopes the device completely).
EDIT: one thing to bear in mind, though, is that the Mini does not have a card slot - so should anything go wrong, or should your dad need a factory reset, all sideloaded books will have to be reloaded manually, but I'm sure you know this already.