I bet the gentelmen is telling you to get a lower yourself because it is the only part of the gun that will be registered to you. He will be able to buy the rest of the parts for you with out them needing to be registered in your name. Shouldn't be to hard to find. I won't be able to help much on the price. Check some of the websites. AR-15.com and the like. Good luck we want pics when your done.
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Only two or three companies make lowers for everybody. The final machining may be better on some than others but I wouldn't fret a lower from Bravo Co, RRA, Colt, Stag, DPMS....see where I'm going? rdl65 is spot on about the cost.
He wants you to buy the lower because it is just like buying a complete firearm. Lowers have serial numbers so it's considered a firearm under federal law. Any lower purchased online must be shipped to a FFL holder. They won't ship it to you unless you have an FFL.