I'm at school at the moment so I can't give you exact code, but I did something like that before;
First, for the turning, you'll have to check if there's an enemy nearby; I'd use c_scan for that, it returns the you-pointer you need. Then you'll have to turn the player, what should be easy; take a look at vec_to_angle in the manual. Last thing to do are the animation and the circle.
If you don't want the player to be able to walk while aiming, you simply can blend into the "aim" animation (ent_blend).
The circle should be a panel, take a look at vec_for_screen in the manual.
I'll try to give you an example right when I'm back home.