Simon, the A, C & K codes refer to the engine( power) and are just a few of many different engine types spread over the Mustang range. The 65/66 series of Mustang seem to have been the most popular however the 67/68 are becoming quite popular recently. The 67/68 have more room under the bonnet for larger engines to be more easily fitted. Obviously a GT is more desireable and collectable but you also need to decide if you want a Coupe (Hardtop),Covertible or Fastback as well. Have a good look around and talk to some owners to help with your decision. Also its gunna cost you around 3 grand to get a car here from LA. Luckily parts and info are readily available for you to do what ever you want. Good luck to you in your quest.