TRAP SHOOTING - -
I've gotted too old and semi-disabled to still shoot trap but I've sure busted a lot of caps at the trap range in my life. I'm going to advise you to visit some shoots and look closely at what everybody shoots.
As I remember, any formal or serious shooting was pretty much all done with the 12 gauge guns. Of course, I may be really out of date and there may be a 20 gauge competition class on today's trap ranges.
As for the gun, be it 12 or 20, all you need is a Remington 870. There are more expensive pump guns but, IMHO, any amount that you spend above the price of the Remington is being spent for bragging rights. I think mine was an 870T Wingmaster. I can check, if needed, because one of my sons has it now.
You'll enjoy the sport if you keep it as a sport. It often becomes an obsession.
GrayBear