Like JPyle, I am a HUGE proponent of building at least the lower receiver to anyone who is interested in the platform.
The lower receiver is a piece of cake to assemble, but it will give you a vast knowledge of what parts do what, so repair/maint/care are much easier on you.
It also allows you to make purchase decisions based on the stuff you want on the weapon as opposed to whatever parts fit into some bean counters mythical figure of what the weapon system should sell for.
Take a look around the AR here and read some of the build threads, many have pictures and great detail, and I think you will be money ahead in the long run.
JD