One paradigm is to think of this as a component system.

Like with living room furniture, you can buy a set or get it piece by piece.

Start with something off the shelf towards the mid-low end like a PSA. Immediately buy a different upper, like a longer barrel upper also in 5.56, or if you get a longer barrel to start, a shorter upper in 5.56 or .300 AAC or 7.62x39. You should be able to get this done for the $1200.

If you get comfortable swapping uppers, the whole gun starts making making more sense as a system. You can have a 16" 5.56, and a 20" 7.62x39 upper that shoots your boyfriends ammo stockpile and you will have a lot more ammo options as well.