That’s a good question.
It’s one I never want to have to use. I will not spend a lot of time shooting it (too much $$$). It’s for 2nd Amendment purposes.
My husband suggests a Bushmaster. An AT-4 carbine with a flash supressor. ‘Course, he’s not sure what your buget is...
A nice WW2 British Enfield .303 shouldn’t run to much, around $200.00 Pretty cheap to shoot and you’d have a nice piece of history.
Given those circumstances, I would look at something in the Kalishnikov family (AK variants). The ammo is fairly easy to get, as are after market accessories, and operator maintenance is a bit easier than the AR series. Speaking in general terms it's not going to be as accurate as an AR, but the larger caliber should be a better defensive round. If you are in an urban area where overpenetration might be a problem, and you most likely wouldn't need to shoot at distances over more than 20-30 yards, you might want to opt for a good 20 guage shotgun (12 if you'd plan on a little more time at the range to familiarize yourself with it.)
Just some thoughts.
You can get a 500 round case of Lake City .223 at sportsmansguide.com for $220. (Russian Wolf brand is cheaper but I do not recommend it).
If price is an issue, you are far better off getting an SKS or AK for hundreds of dollars less than an AR-15, and use the money you save to buy ammo. If you do not practice with it until it becomes second nature, you will just freeze if you have to use it under stress. And AK ammo is also cheaper than .223