Are you interested in a rifle or rifle-caliber pistol? In a rifle, just be sure to get a 16” barrel. In a pistol, anything longer than 8” is sufficient. I only mention this because lots of uppers for 300 blackout are less than the required length for a rifle. If budget is a constraint, Palmetto State Armory is a great place to start. You can buy a complete upper and complete lower, have them ( or at least the lower) shipped to your ffl, slap the halves together and be reasonable confident it will operate just as it should. For a firearm that will not see too much use, I think it would suit your needs