I'd get an M1 Garand from this place www.thecmp.org/m1garand.htm
I would find the cheapest CETME I could, looked over thoroughly to insure headspacing and everything was in order, buy tons of mags for it because they're cheap. Buy ammo with what's left over. A mosin and several cases of ammo is probably the best idea, but semi auto is king when it comes to any kind of defensive stuff.
mosin nagant with a scope and 10k rounds of ammo.
You could fend off alot of rabid squirrels with that set-up !A worked over Ruger 10/22 with some Ruger and Butler Creek mags... Hogue Stock, better receiver, barrel, trigger group... 10,000 rounds of ammo... mainly because if standing ground it should last a bit and if one had to go mobile it would be fairly light to carry...
Mags can make a huge difference in over-all affordability and should always be factored in. A $500 AK with 30 mags ($9-$20 each) costs almost the same as a much nicer $900 PTR-91 with 30 mags ($.99 each)!
And how the hell is one person supposed to carry 30 mags around... :-)
I'm not looking to buy anything, but hypothetically speaking, if you had a thirst for a rifle around $500 OR LESS, and one rifle only, what would you buy? Now that doesn't mean get 2 of some cheap rifle for $500, but instead only one.
For example, a cheap saiga 12 gauge or an AK in any caliber. Maybe a GSG of some sort.
Wow surprises me no one mentions a .22.