Its a car, it has an engine that moves it, it will do miles as the engine don't know the difference between 1 mile and 1 hundred miles, speed is not everyones things, as long as you are safe on the motorway then job done
He's obviously more interested in driving his modern car on the motorway, so be it, offer him a lower price to take it off his hands
A mini is fine on the local roads as well as the long roads
Only thing you need to watch out for is this:
and watch out for getting blown about when overtaking / being overtaken by HGV's.
This can be daunting at first but thats not just a mini thing its a small car thing, that might be what he meant but its certainly no reason not to get one
You'll soon learn to forget about your creature comforts you are used to at the moment...I hope