To properly do a barrel to upper receiver you will need the following:
AR 15 armorers wrench
Pin Punch to set the gas tube
Vice block to hold the upper receiver in place as you are attaching the barrel locking nut.
Torque Wrench to set the barrel locking nut at the proper weight
A MATCHED set of GO and NO GO gagues and they should be the same as the caliber of the rifle...ie 223 chamber or 5.56 chamber
do a search on youtube and you shoud be good to go if you are handy with tools.
If after watching the vidoes if you thing you can do it then go for it
If not then either take the parts to a gunsmith or get a fully assembled upper reciever with a MATCHING bolt carrier group.
If not a good gunsmith can make it happen for you just ask them if they are a AR 15 armorer or if they have done AR 15 builds before.
The Valley Forge show there is a guy there that sells AR barrels and uppers and if I recall right they can do the job for you.
good luck either way you decide