You might want to take a look at some of the DPMS offerings, I think they have a sportical in .308. It is a bare bones AR, but they are supposed to be reliable. They also take the new 20 round PMAGs in .308.
As far optics, are you looking for combat accuracy at 500 yards where you can hit a man sized target reliably (then I would stick with something low powered or iron sights), or precision accuracy when you start needing those high powered scopes? Or both? In which case something like a 2.5x10 is handy, since you can use is low for close in, and still make far shots.