Given your constraints I would lean towards the SKS. This wouldn't be my first choice overall, but it rises to the top for your particular situation!
Here are my reasons, based on your limitations:
1) Your price range is a limiting factor for a defensive rifle. If you were willing to spend more money, I'd lean you towards an AR-15. The AR-15 is accurate, reliable, shoots common ammo, and the parts/pieces/magazines/etc are easy to find. The gun is also lightweight, portable, and durable. But, prices for a used rifle would probably start at $700 these days (just guessing off the top of my head).
An AK-variant is also a nice choice, and these weapons are extremely reliable... Mind you, they aren't as accurate as an AR-15, but they'll get the job done just fine, and the ammo is easy to find. Again, price may have become a limiting factor for these rifles in the post-election frenzy. In the past you would have been able to find a reasonble quality AK for under $500, but I'm not so sure how things are looking on that level at the moment!
2) Your recoil sensitivity is also a limiting factor. If you weren't recoil sensitive you could certainly get a nice (brand new) defensive pump shotgun in that price range, and still have money left over for ammo and accessories. The 12 gauge shotgun is an extremely effective short-range weapon, and does well for double-duty use as a hunting weapon (as a kid I hunted everything from squirrels, to pheasant, to turkey and deer with a 12 ga shotgun). But, the shotgun will hit you a bit harder than the AR/Mini 14/SKS type choices. Still, depending on how recoil sensitive you really are, this may still be a viable option (look at different pads, reduced recoil slugs, etc). You may also learn to be less recoil sensitive with practice!
Anyway, the SKS is not without advantages. The last time I priced one they were still available for less than $250 (though it has been a year or so). The ammunition is identical to most AK-variant rifles, and is easy to find in bulk. These rifles are also extremely reliable, and I've put many thousands of rounds through one of mine without a single malfunction (even while shooting very dirty ammo, through a very dirty rifle). One limiting factor on a stock SKS is that the magazine is built-in, and only holds ten rounds. Still, the rifles are stripper clip compatible, and reloads will go quite quickly with practice. Some folks modify the rifles to take detachable magazines, but I'd just push for an AK if you were thinking of going that route! Both the SKS and AK certainly have a bit more recoil than the AR-15 rifles, but it is not ridiculous by any means (probably still less than the 20 gauge slug/buckshot loads).