I second the Remington 870. I also put a Choate extension tube on mine (+2), and a short, 18" barrel. I think that the combo works best with a plain bead up front. this is as fast a sight combo as you can have, and you don't need to line anything up at less than 10 yards. Just put the bead on it, and it is destroyed. I can't see the use of a peep sight, unless you want to use the gun at some distance. For home defense ranges, the bead is as fast as you can get. The shotgun is a close range tool, but if you want to use it at a greater distance, then I expect that some sort of rear sight would be useful.
I happen to like peep sights, or I used to when my vision was better. They work better and faster than open sights, at least within most useful distances.
I recommend 12 guage (or 20), but definitely use something like Federal OO tactical buck in a 12. If you are in a warm climate, then shot might be OK, but I wouldn't trust ammo designed to kill birds in a place where people may wear heavy clothing. As distance increases, the pattern opens up to compensate for sighting innaccuracy. You do have to put the bead on it, but the combo of a 12 guage with 00 tactical Buck is great out to 50 feet! Pattern the stuff, and you will be pleased.
Check out your ammo to be sure that it doesn't hang up when you shoot. Some ammo is not reliable in my gun, and you can't tell by looking at the shells. You have to fire them to know.