I understand the desire for the lightweight lower receiver, but the difference between the aluminum and composite lowers couldn't be more than a few ounces given the light weight of an aluminum lower anyway. That being the case, I'd just go with an aluminum lower. As someone already mentioned, Palmetto has nice prices on their lowers and parts kits. For the cost of just a few ounces more, I'd definitely go that route. I've used Essential Arms, Bushmaster, Stag, Cavalry Arms, and Spikes, and all of them so far have had a nice ft and finish.
As to a lightweight upper, I *think* Bushmaster's 16" super-light barrel is the lightest one out there, but don't quote me on it. (bushmaster part number A BBL-16SLA) . That's what I used when I built a rifle for my oldest son. He took a deer with it last year and he also used it at an appleseed shoot this past May. He's 9 now, btw. Without the scope on it it's a darn light rifle and when I swap out the aluminum lower for the Cav-Arms lower I have on it, it really doesn't change the weight all that much... maybe a few ounces, but not more than that.