What is the gun for and How much can I afford?
Firstly, you have to decide EXACTLY what you are going to be using this build for and exactly how much money you want to spend on it. This forethought is imperative to keep you from spending $1000s on a gun that isn't exactly what you want. The largest problem that I've run into with building my own AR-15 rifles isn't acquiring new pieces or parts to the build, but being able to sell the old parts for a reasonable price. The current armorer's market is not friendly to people trying to get rid of old gun parts... That being said, it is important to know exactly what you parts you want to include on your build and stick to that plan.
Short Barreled Rifle or Pistol Build?
With the full velocity and accuracy potential of the 300 Blackout round being realized in a 9" barrel, there is no need to meet the 16" barrel restrictions we're so familiar with the .223/5.56 AR-15 necessitating. This means that we can throw the pistol/rifle laws out the window, Right?! WRONG! With any rifle under the 16" barrel mark, the item needs to be registered as a Short Barreled Rifle through the same process you'd purchase a silencer through. I once again suggest an NFA Trust as the entity that you register the SBR through. The only alternative is to purchase a firearm that has been built, from the beginning, into a pistol, or building a new lower receiver into a pistol. A pistol is defined as a firearm under 16" barrel length that is meant to be fired with one hand. This means that you can't put a butt stock onto a pistol build. This becomes a Short Barreled Rifle and one could incur quite a few legal ramifications if the ATF discovers you've altered your pistol into a SBR... Not advisable. There are products like the SigSauer SB-15 brace that are meant to facilitate the one handed firing of an AR-15 pistol and the ATF fully permits the use of this item in pistol builds.