That all assumes that you buy a barrel which already has the barrel extension installed and headspaced. Better barrels are headspaced to a specific bolt which is ether suppled by the barrel manufacturer or by the customer. Headspacing an AR-15 requires some machining.
It also assumes that the barrel has been contoured and the gas port has been drilled by the barrel manufacturer. Determining the gas port location and diameter is generally taken care of by the barrel manufacturer, but may not be the case if your barrel is for a wildcat chamber or special loads. That too requires some machining skill to drill the hole, but far more important is the skill to determine the proper port diameter and location for a particular cartridge.
Just installing a barrel can be done with just a vise, an action wrench. and a torque wrench (optional if you're good at estimating torque) and allen wrenches for tightening screws on the gas block. Installing a new pinned gas block requires machining.
AR-15's were designed to have their barrels replaced in the field by military armorers with only simple hand tools. The military suppled barrels have the barrel extensions headspaced and gas blocks (front sight) already installed on the barrel Check what you're getting when you buy a barrel.