I'd start with a complete gun with a 5.56 upper on it and then add uppers like the 300 or whatever else you want.
Ding ding ding, we have a winner. Start with the basics like a contender and add barrels, or in this case uppers. ARs are easy to build and much less costly when rolling your own so to speak. I have built 6 over the years and have tutored several friends in building their own. Not only do you learn how to build it but you learn how to repair it. And building it yourself, the options are limitless.
