Why not just stick with the basic Colt M4 type AR15 since you are already experienced with it. They can be had for anywhere from $1000 on up depending on exact configuration. I provided a link to a pretty nice model that comes equipped with some aftermarket options just as an example. This would leave you plenty of money for the ACOG, additional magazines, ammo and any other desired bits and pieces.
Good luck and let us know what you decide.