'extras' depends also upon what you want the gun to do. Is your purpose just recreational shooting, claps/trap, home defense, hunting (if so, what?), or all of the above?
But basically, what Dave said. Burn a few thousand rounds through it, and maybe take a general shotgun instructional course. You'll hind that shotgun shooting and rifle shooting look pretty similar, but are just different enough to matter.
Mike
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"A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects." -Robert Heinlein