The lower receiver requires a transfer though an FFL, so most people seem to start there. However I have heard there are not a ton of lowers around, maybe because of orders for the S&W AR?

It really makes no difference what order you buy them in. Are you going to buy the upper complete? If you don't, you need an action block, and some other stuff to put the barrel into the upper. I think Teknic's price on Stag uppers is a good deal, but I don't know if he has any in stock or not.