Have you seen the monthly subscription plan for building am AR? Ready Gunner does it and you can set it up to work upon different time frames. Then every month you get a part or 2 and when the subscription is complete you have a complete rifle. Then you could build on the cheap while getting ammo too.
If it is gonna be a range gun, and you want to build and not buy complete, buy a little at a time while buying more ammo. The 2nd does not protect ammo, so eventually they will go after that.