Please keep in mind Cerendrad, that if this is your first AR, then consider becoming comfortable with the iron sights before delving off into optics. Optics are a blast, tons of fun to talk about, buy, shoot with, but there is no substitute for training. You can, and should be able to make the shot that you are talking about at 300m with only the iron sights. The optics are a great tool, but hitting your target at that range and much, much further has been done for a century using only skill and iron; something to think about at any rate.
As for my vote on a good optic for the AR; it would be a Trajicon Reflex sight. It can be dialed down fine enough to makes target acquisition, and follow-up faster, and at a 300m, the .223 don't need no stinking magnification, IMHO. It also doesn't break the bank.