As far as I know the only way to get the mustang is to buy the collector's edition, but you may be able to get it through completion of the game.
Police vehicles are awarded through the reward cards, which are fulfilled by completing certain objectives throughout career, challenge series and quick race. They are sort of like milestones in Most Wanted, only these aren't just restricted to cop chases. You can get all sorts of rewards from them, including aforementioned cop cars, career cars, rims, vinyls and spoilers for auto sculpt.
Murcie is back.