To build the full upper to be able to just swap one out for the other (transferring bolt, bolt carrier assembly, and charging handle), you'll need:

Receiver, barrel nut/delta ring, crush washer, weld spring, handguard, handguard cap, Gas tube, Gas block, gas tube roll pin, gas block, and barrel.

Or alternatively, if you install a free floating handguard (which usually come with their own barrel nut), and which I highly recommend!!!, you would need:

Receiver, free floating handguard set, gas tube, gas tube roll pin, gas block, and barrel.

If you get a stripped A2, A3, or A4 receiver, then you will also need the forward assist assembly and ejection port cover assembly.

No worries though, I'm not trying to scare you off, I just don't want you to be disappointed when you're short some parts when that big box arrives!