To add to what Ray mentioned: here's a question....

do you have the requisite knowledge to understand the small but significant differences between twist rates, gas systems, buffer weights, etc. to really know what you want out of the gun.

If I were to give one piece of advice for this, it would be this: work with a well qualified gunsmith and tell him/her what you want the gun to you. Use a smaller local business. You will DEFINITELY get exactly what you want.

I'd suggest BP Tactical.