Nearly any gun will suffice provided it is 9mm or larger. You need a holster that covers the trigger guard of the gun and magazine pouches for the magazines. If you plan to shoot production or Limited 10 you will need five magazines, carrying four on the belt. If you plan to shoot single stack six on the belt is recommended. The extra magazines are the first hang up for most people.
United States Practical Shooting Association is a good place to check out the rules and the divisions.