first, i think in SHTF situation, for defense, you dont really need to reach out 500 plus yards. There are few situations even in food gathering where a 500 yard shot will ever present itself for most unless you live in the desert.
While im not saying, you dont need a 500 yard rifle, if you are looking for a worst case scenario rifle, you need an autoloader, the faster to load and higher capacity the better.
You said you wanted an AK or SKS, and i think those are excellent choices. Here is a link to a site with a nice romy military variation AK with a longer stock to fit most american shooters. Its only $350, which should fit your budget. Shipping should be under 20 bucks to your FFL dealer for a transfer.
Classic Arms Inc.
scroll down until you see ITEM # ROM-AK MV-BPS.............$349.95
Another option if you want to extend your range, is to save for a little bit and pick up a mosin nagant for around $100 bucks. Youre going to need to be putting money aside for mags and ammo anyway, might as well drop 20 dollars or so a week into a gun fund and get yourself geared up...
these two rifles are rugged, reliable and should cover your needs.