We found it easier to look for the car we wanted rather than dealer we wanted, as most used car dealers have a fairly limited stock. With what I know now I would probably have bought privately though, but when you first arrive the registration and WOF process is an unknown.
As far as warranty is concerned, we had an issue with our car in Wellington, having bought it in Auckland where the dealership provided us with a "return to base" in-house policy, and they STILL honoured the claim (repaired in Wellington) without any problem whatsoever. And we're talking about $1700 claim on a $6000 vehicle. So even though our policy wasn't national at all, they honoured it. I think most typical underwritten mechanical warranties are specified as NZ wide, which certainly would cover you nationwide.