They make them in mid-length, I have one in that setup. I agree, the BCG and barrel are the most important parts. Also, the barrel isn't as easy to swap later like some things are.
You can find a good quality barrel for about $180. If you have a budget, thats where I'd be looking. No need to go over $200 unless you want a comp long-range AR. Those 24in SS bull barrels get pricey!!