Go w/ 5.56/.223. I ran 300 BLK for a while but ammunition is quite a bit more expensive and not as readily available. Plus it's made for shorter ranges. The system/round was originally designed to be shot out of a SBR at CQB ranges.
$1500 will get you a lot of rifle. If your budget is $1500 I would look to spend the majority of it on the AR itself. I would not recommend spending $600 on a rifle and the remaining $900 on UTG/Macko junk. I would shoot the rifle as is and figure out what you want to mount on it as you figure out what works best for you. If not the chances are you'll end up wasting money that could have been spent on ammunition and other necessities. Avoid the initial buying spree.
Look at quality, value packed options such as BCM, Daniel Defense, and Colt. These companies produce a consistantly reliable AR. You can have the piece of mind that comes w/ purchasing quality.
Look into the light barrel profile as well.
BUIS, Troy or Magpul. Why 45deg?
A good 16" barrel will cover most bases.