If you buy the upper complete you'll save the more complex assembly stuff and a large portion of the AR specific tools.

Lowers require a hammer, a way to drive a couple roll pins in, a screwdriver and a wrench.

If you buy a complete upper and a stripped lower you still need a buttstock assembly (buttstock, buttplate, screws, sling swivel, lower receiver extension, buffer and spring)  a lower receiver parts kit (FCG) and a pistol grip.

With the exception of the roll pins for the bolt catch and trigger guard and the LRE you can assemble a lower with your bare hands.

All the info is here along with the manuals.