In that case, I'd recommend either a 3x9 or 3.5x10 variable power scope. A basic Leupold VX-II 3x9 40mm with Duplex reticule runs about $260, and a high-end Vari-X III 3.5x10 LR Mil-Dot M1 or M3 costs $800 or so, with many models inbetween. As you would imagine, the more high-end features you want, the higher the price.
Anyway, something in that range will cover the shooting you want to do quite well. Buy an Armalite one-piece mount and you're good to go with whichever model you choose. You don't need to spend a ton of money, but I've been burned several times buying "bargain" scopes, and I won't buy anything less than a Leupold for anything anymore. I've never had a failure with them, but on the remote chance that I do, they have a no-hassle lifetime warranty that they are well-known for honoring.
-Troy