Best bet as far as choosing a shotgun is to go check out your local 3-gun matches and see what the better competitors there are using. It takes a REALLY good shooter to be limited by the equipment, but if you're serious about this there is no reason to handicap yourself from the beginning. Might as well spend your time and money learning the platform you're going to run over the long haul from the beginning, and stick with it.
Mindset - Skillset - Toolset. In that order!
Attitude and skill will get you through times of no gear, better than gear will get you through times of no attitude and no skill.