Smith & Wesson M&P Sport II should do the trick. Great rifle for $450-500.
You know what, I take it back. Look for an AR that has a mid-length or rifle-length gas system. It will be more reliable and recoil less. A nickel-boron bolt carrier group is also worth the extra $30 or so. They clean up much easier and tend to function more reliably if the bolt starts to get a little dry.
Here’s an excellent deal on a decent upper receiver that can be shipped right to your house:
http://palmettostatearmory.com/psa-...upper-with-nickel-boron-bcg-ch-516447379.html
You won’t be picking flees off field mice but it’ll work for plinking and coyotes. You can always swap the barrel later for $1-200 if you really want to.
You can purchase a complete lower receiver online but it has to be shipped to an FFL who can process the transfer (usually $20). If you’re unfamiliar with buying guns online, stop by a gunshop and ask, or just see if they have a complete lower receiver on hand for a couple hundred bucks. You usually don’t need to have an upper and lower that are the same brand because the dimensions are universal. (This is NOT true of large frame ARs in 308, 243, 6.5 creedmoor, etc.)
I almost always buy separate uppers and lowers. It’s often cheaper than buying a complete rifle.