You can find a good Mossberg M500 for a couple hundred bucks. The Mossberg Maverick 88 models are the same gun as a M500 with a few (mostly cosmetic) differences, and I prefer the safety location on the Mav88 over the M500 safety. A good used Maverick 88 by Mossberg is well under $150 for a good used example. Don't screw around with pistol grip stocks or forends; a good polymer buttstock and standard forend with an 18.5" barrel is ideal. Most SD shotgun use will be 6' distance and under so don't worry about laser sights or anything fancy. Just point and pop. The Mav88 mossberg is actually a no frills M500; the forend is slightly different mechanically but functionally more or less identical. You can't go wrong with a Mossberg.
By the same token Remington 870s, Winchester Speedpumps/1300s, etc are all good choices. There are those who will say, only buy an 870 because that's what the cops use, or only buy a Mossberg because the Marines use them... Well, a lot of people buy Fords, and a lot buy Chevys, but they both haul the mail adequately. Buy what you can find the best deal on and like the most. Check out safety location, feel of the stocks, etc. and get what you want. Buy a box or two of tactical 00 buckshot, and a half-a-dozen boxes of cheap birdshot (I like #6) and practice with it.
A good quality bright flashlight should be kept near the shotgun. Check the batteries often and don't let the wife/kids "borrow" the light, treat it like a fire extinguisher, need to know where it is at all times.