Hi Tom
When you're looking for a head unit, you can go for single or double din, any should be fine, looking for one with a touch screen or simple with buttons? when I was getting (you won't like me now) subwoofer fitted and head unit I heard a lot of people say that the Bose sound systems are annoying to work on (I haven't got a Bose system, so people was going "phew").
So as Trevor has said check it out with a specialist, it could just be down to a few connections, let us know how it goes :)
Regards
Bradley