UncleBuck,
Look for a new or lightly used Mossberg 500 or Remington 870. Try to get one with a 20-22" barrel. Any longer would be difficult to maneuver around and through doorways.
You want to consider what it has for a choke in the end of the barrel also. The choke controls the spread pattern of the pellets of the shot. There are various chokes available but I would suggest for you modified (half choke), quarter choke, or improved cylinder. Do not get full choke unless you are planning on killing something at 50 yards.
You will want to get buckshot when searching for ammo. #4 buckshot is my personal preference over 00 buck. You dont want birdshot..
Like previous post have explained when your pointing that gun in the intruders direction do not hesitate or try to intimidate. When you have a clear shot, take it. Trying to intimidate someone with a firearm could be worse than not having a gun. If they see the pussy in your eye they will try to kill you. Also shoot to kill, a wounded man will stalk you and kill you fueled by revenge.
In my state you can kill a person if they are a threat inside of your home. Be prepared to go to jail because you did just kill someone lol
I suggest reading "Survival Guns" by Mel Tappan, "Holding Your Ground" by Joe Nobody and "Total resistance" by Major H. Von Dach.
Hope this info helps, I gotta smoke this OGKush that just walked through my door!!
Edit: Get one with a shoulder stock and not a pistol grip. Pistol grips look badass and all but wouldn't be practical for your situation. A stock will be more accurate and gentle on the wrist