All great answers and very helpful info. I am looking into getting into 3 or 2 gun competitions and was leaning towards just buying an off the shelf rifle from a good company to start with. I know I can build one cheaper but I want it to run like a top as well. I feel that the only thing that matters in the lower is the trigger. After that most everything is roughly the same besides furniture. Keep the suggestions coming this is some great stuff.
Go to a few matches before investing in comp gear. I had similar intentions and my schedule keeps me from even going to spectate most events. An m&p sport, M&P 9 and a decent pump 12 ga will get you in the door of a three gun for less than I see a lot of wannabe multigunners (like me) have in just the ar. I regularly shoot pistols now and rarely pull out any one of my 3 ar's.
Personal experience and use will drive what is most important in an ar to you. I have built 5 sold 3 and bought one. I now wish I would have just bought a nice Daniel defense with a midlength gas system. I will never see the money back out that I have in my homebuilt ar's, I enjoy them but I wish I would have put the extra cash into my pistol fund, or ammo.