Far more important than the gun and accessories is the skill of knowing how to defend your home and family.
And that has little to do with the choice of gun.
Now, that I've gotten that off my chest, you can, indeed, outfit yourself for under $500.
There's tons of good, used Remington 1100 autoloaders out there, in the $300 to $350 range.
Even less if you don't care how pretty it is and if it has a fixed choke barrel without a ventilated rib.
Add a mag extension and the sights, and you're well under the budget.
Lock the doors, they're coming in the windows.