Take home message- the AR platform is very user serviceable. You can get a basic AR - put whatever you want into it- trigger, furniture, sights. Don’t go really low end and don’t break the bank.
Essentially you have a 7075 aluminum lower with a lower parts kit. Add a 16” upper.
1) Lower can accept any upper as long as you follow NFA rules
2) The addiction will soon create additional purchases. This can be accomplished by buying additional uppers which don’t require a transfer through an FFL. Think of it as a barrel swap. That means you can get a 300 Blackout upper or a 22” varmint upper.

Colt 6920.