The supply of the unfinished lower receivers has pretty well dried up from what I can find.
The question to be considered is the act of drilling the receiver blank illegal in the state you live in. It is my understanding that in most it is not.
The only way I would be interested in building an AR type rifle is in 308 caliber. I am not at all interested in a 223. In my opinion the 223 is too light a caliber for anything I might use it for.
A heavy bullet for 223 is about 70 grains, 308 is commonly loaded with 180 grain bullets. There is a huge difference in impact energy with the 308 180 grain load than the 223 with 70 grain.
I somewhat agree with you. My next build is going to be an AR-10. I have a .30-06 hunting rifle and a .303 Lee-Enfield that will suffice for the time being. But one of the nice things about the AR-15 platform is reloading wicked fragmenting rounds. It is difficult to get the velocity up on a 180 gr. 7.62 round to make it fragment. You can get knock down power and expansion with 7.62. With all the body armor around these days, you need to think "hips and head" when targeting. Hips are full of arteries that are just waiting for fragmenting rounds. It immobilizes the target with a quick bleed out. I'm not saying that similar or better results can't be achieved with different targeting using a .308. I would say, I would rather have knock down power if a drug crazed target was rapidly advancing on me.