For what you want just get a 5.56/223 one.
Same as the 9mm get the common one with cheap plentiful ammo.
The suppressed 300blackout with subsonic rounds sounds pretty cool though too! So many options!
Guess I have to pay $400 to NSA for a suppressor that really sucks....
https://www.aeroprecisionusa.com/m4e1-sd-75in-300-complete-upper
Have to go do the NFA process and engraving I believe? Really sucks because this gun would be awesome.
If that is your budget for the upper and can you can do much better than Aero precision. Aero is great for budget minded stuff, but once you skip $700-$800 for a rifle from them there are better choices.
That one is just high because it has permanent silencer built in. Seems like a pretty good deal considering the silencer is $900 alone. pretty cool being just 7.5" with the silencer.