For Home Defense, it is my opinion that anything over 20 gauge should be fine. Mossberg makes one already put together, so you do not need to buy and modify anything, I am a fan of factory built anything. 18 or 20 inch barrel, and a collapsable stock. Makes it about 36" long collapsed for concealed storage. I am not a big fan of pistol grips, but, on a collapsable stock, that is what you get.
In all the reading I have done, I think "T" shot is the load of choice, and this is what I decided to stock up on. Some say 00 or 000 buckshot, but inside it will make a real mess of the house and perhaps harm those in the next room.
Were someone to attempt to invade my home in the middle of the night, after identifying that it was not my son coming home late, my first goal would be to injure not kill. At those close ranges 5-10 feet Bird Shot will not spread more than 5" anyway, and a shot to the abdomen or groin should do the trick. Some say that that shot is no good because it does not penetrate, but it will certainly knock someone down.
Over/Under, Double barrel side by side, or semi auto, followed by pump, and single shot last choice for me. Only the movies need lots of rounds. 1 or 2 well placed shots should be enough.