For a starter shotgun that's best suited for home defense I would suggest the Mossberg Maverick.
It comes with a shorter barrel (18 1/2 inches or 20) and larger tube for shells (6 or 8) and can be found at Academy Sports for under $200.
There isn't a better bargain for a gun then this.
http://www.maverickarms.com/pages/88security.htm
EDIT ----
It also shares a lot of the same parts with a Mossberg 500. That means you can swap out barrels and the stock. So you could get either a pistol grip with an adjustable stock, or a shortened traditional stock. This would reduce the reach your forced to make and should improve your maneuverability with the gun.
+1 best buy for the money.