Now is a really bad time to buy an AR. Availability is in the crapper, and prices are sky high.
Depending on how much of your rifle you want to actually build yourself, you may need some extra tools. The castle nut on a carbine stock and the barrel nut on the upper both need to be installed and torqued correctly, or problems can result. To do that requires an AR-15 armorer wrench, a proper bench and vice, a torque wrench, and receiver blocks. If you don't already have most of those items, that can result in a significant investment.
If you buy a mostly or completely pre-assembled upper and lower, swapping some smaller parts (trigger, stock, grip, etc) are quite easy, and require only basic hand tools. That kind of work is pretty much in line with swapping Glock frame parts around.
Never argue with an idiot. They will drag you down to their level and beat you with experience.