Get a good used Remington 870.
Straight forward operation, easy to learn and operate, breaks down for cleaning and goes back together easily. It suits the purpose you stated well. You probably will never need to fix it, but if you do all the parts are readily available and every gun store is familiar with them.
Mossberg 590's are decent, they have had some trouble with the safety in the past. They hold a tight pattern.
Shotguns can put a lot of rounds down range in a hurry. 000 buck rounds have eight .36 caliber pellets in each round (in the popular 2 3/4", more if you go up to 3", but probably not necessary), that's a lot of lead heading downrange at once. With a little practice you can get several rounds off in two seconds. A 1 once slug is another option, and is generally a one shot assailant stopper.
Rifles rock, but for close up work, shotguns kick ass.
Edit: I forgot to add that EVERYONE speaks shotgun.