I have not built any AR's yet. I do know that with the stripped lower you will need all of the parts that go into it. There are quite a few folks on here that realy know the poop on the AR and should be helpful.
From what little I know building an AR is like builing an engine in the that fact that you have a few big choices in parts, and a ton of smaller ones. Good luck I hope it turns out well for you.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.