Anyone can build an AR-15. Not everybody can build a smooth running and straight shooting AR-15.
I can put a complete AR-15 together in about an hour from parts - but that doesn't mean it'll actually run properly. I spend a minimum of 10hrs, usually 15-20hrs fitting and tuning when I actually build one - and professional, full time, custom build guys like Ritch Johnson or Mike Milli end up with a better product than I can produce, simply because they have even more equipment than I do.
But to answer your question:
Originally Posted By: zr600[W]ould it be just as accurate if i got just a barrel from him and had my buddy build the upper for me[?]
Most likely, Ritch would produce a more accurate product. Now - that assumes that you're a proficient enough shooter to capitalize upon that difference. If you're not capable of shooting better than 1-1.5MOA, then it won't matter whether the rifle is 1/4MOA capable or 3/4MOA, you won't be able to coax it to do anything better. Blackhole barrels shoot great, and a trued receiver and matched bolt with a blackhole barrel will shoot even better. In general, self builders without a machine lathe and a selection of bolts to choose from can get within about 80% of proper builders, at best.
Nothing wrong with building it yourself, but don't be fooled into thinking that you'll be able to match their products after "building lots of AR's" - Lots is a relative term, I've built over a hundred of them, and used to true everything on a machine lathe when I had access to one, even at that, I can't keep up with a guy that does it full time as his livelihood.