Where I am confused is the barrel,  when you buy a barrel does it come with the barrel extension installe/timed,whatever else is needed?

Just buy an upper from Bravo Company. They have a great special running right now.
It will be headspaced, tested and ready to put on your lower. Can't beat it.

Most barrels come with the barrel extension properly installed. As for parts you will need:



-Barrel

-Gas block/FSB ( depending on how you are building it)

-Proper length gas tube

-Upper receiver( if it is stripped you will need the forward assist assembly and ejection port door assembly)

-BCG

-Charging handle



Those are the bare essentials, don't forget about handguards and some sights.



EDIT:   The BCM uppers mentioned above are great.  I wouldn't hesitate to buy one of those instead of build my own.