I would recommend buying a complete factory AR for the "first" one. READ a lot, dozens of manufacturers and styles. Do you want a carbine or rifle? Is meeting the mil-spec important to you?
I disagree with the first statement, but agree wholeheartedly with the remaining ones.
Building your first rifle is a great way to understand the operation of the AR and how everything goes together. If you are smart about it, it will
save you money, and there is a lot of pride in knowing you built something. I built my first one (and all subsequent ones), and I wouldn't do it any other way (unless a great deal fell in my lap). The lower is pretty easy to put together, and it's not hard to find assembled barreled uppers with lifetime warranties (I used an Spikes 16" M4LE upper for my first build).
But yes - research is a must for any AR purchase. Read up!