12 ga so you have the knock down power, and pellets for plates and angle iron pieces at distance.
Semi auto. Because they're faster and less recoil, especially if a gas gun.
Barrel, 21 or 22". Shorter = less weight.
Choke - yes. LM, IC, and LF will cover just about everything you'd encounter. A diffuser choke is nice when you have juicy clay targets...
Ammo - yes, you'll want some of that too. Low velocity, 7-1/2 shot works. You'll need to experiment with loads/brands to see what cycles in whatever gun you get.
My 16 y/o petite daughter runs my versamax like a boss, she just doesn't have the upper body strength yet to run around with it and swing it a lot. But as far as being able to shoot successive targets fast, no problem.