1) 12-guage shotgun. I would recommend a Remington 870 or Mossberg 500. Buckshot ammo: this was you don't have to worry about rounds going through the walls and into your neighbor's house. Also works great when you lose your fine motor function (adrenaline) and is easier to aim in low light.
2) alarm system, i.e. ADT, Brinks.
3) reinforced (solid) front door and master bedroom (and/or youngest child's room) door with commercial hinges. A sturdy deadbolt is a must. This door can be broken down but it buys you several minutes.