gun_slinger_boy
.338 Win Mag
The hardest part of building a lower is not launching a pin or detente somewhere that you can't find it. There are tools made for some of these (for example, the front pivot pin spring/detente) and you can take steps to minimize the chances of losing it, like installing it inside of a plastic bag. Most of the tools needed (other than an armorer's wrench) are in most home workshops. With uppers, the tools that you really need to do a barrel install correctly (1/2" drive torque wrench, 1/2" breaker bar, (re)action rod/block, go/no-go gauges, etc) are less common. Worth investing in if you think that you will (it's addicting, so you probably will) build a few uppers down the road. However, a prebuilt upper on a home-built lower is a good start to the hobby.
I know this will make some cringe but you can actually assemble an AR with ALL common tools. For instance you can use a strap wrench to tighten the castle nut and then a hammer and punch to finish it. Depends how pretty you want your AR to be inthe end.