Complete upper with Bolt, BC and charging handle.
Besides a lower and stock , what else is needed to complete?
Complete lower or stripped lower?
"Igitur qui desiderat pacem, praeparet bellum."
Therefore, whoever wishes for peace, let him prepare for war.
without those its a pain!
what kind of an upper?flat top or fixed handle?
if you have a flat top you might want to consider having a buis-back up iron sight-in the event that your scope gets knocked out of zero
and if it has a flat front sight block -a back up for that too
if its a fixed handle style your golden!
if you get a complete lower you wont need the internals or the stock
most of them you can get with your choice of stocks-state dependent!
check local laws before getting any collapsible ones
but for the most part thats it in a nutshell!ar build 101
and yes get penty of ammo (and a few mags to put them in )