The 870 you have will do the trick. Ghost rings are a great idea because many shotguns force you to alter your point of aim when firing from various distances: Low holds at 25 yards for buck and slugs. Yet, we normally train to aim center mass. With ghost rings, you can regulate the gun, aim center mass and place your rounds on target. Mag extension is a great idea. Spend the money and buy one of quality. Adding a light is a great idea. Either cut down your current stock to fit, or purchase a youth stock to shorten the length of pull. And then do this...
Purchase a couple of boxes of Federal Personal Defense Buckshot. Go to the range and shoot those next to your regular load. It's basically the same as the tactical buck we shoot. It has an incredible pattern, and it's low recoil. You'll likely never shoot regular buck again.