Remington Sportsman 12 magnum 12 guage 21 inch bbl with rifle sights, I usually leave the rifled tube in it. For steels I shoot Federal Trap loads with #8 shot (all of our IDPA side matches are close range so buckshot isn't necessary) and for slug stages I use 1 oz winchester or federal max dram slugs. I use 23/4 inch shells. I have an aluminum extended mag that allows me to hold 8 rounds total in the gun.
Using an "express" model remington or mossberg you should be able to get into this for well under $250.