I think you should buy the 24" from PSA because I want one. I could live vicariously through you
. That would make a killer prairie dog rig.
If you decide to build you basically have three options, either build a lower and buy a complete upper, build both, or buy both. Obviously buying both is the easiest, it takes about 5 seconds to snap an upper and lower together.
Building a lower is very easy. If you can change the oil in your car you're capable of building a lower. It requires a few basic hand tools. The first one I built took maybe an hour since I was going really slow. My second one took about 30 minutes. The brand of lower really doesn't matter, as pretty much all of them are build by one of just a few manufacturers. Pick the one that's cheapest or has the coolest markings. Arfcom has a really good how-to thread on assembling a lower:
http://www.ar15.com/forums/topic.html?b=3&f=4&t=226782Building an upper is a little more involved and requires some specialized tools. At the very least you'll need punches, a hammer, an upper receiver vise block, a bench vise, an armorer's wrench, and a torque wrench. Depending on what handguard you choose you may also need to buy a proprietary wrench from the handguard manufacturer. I'm not totally convinced you can save much money by building an upper. With that said I assembled both of my uppers myself from scratch because I couldn't find the exact configuration I wanted anywhere else. It also allows you the luxury of buying it one piece at a time, so you can spread out the cost.
When building an upper there is some debate about whether you need to worry about headspace. On the AR system headspace is set by the barrel extension, not by the bolt/barrel fit. So, as long as the bolt is in spec and the place you bought your barrel from installed the extension correctly you'll be fine. In my opinion as long as you buy from reputable manufacturers you shouldn't have a problem.
The hardest part (for me) of building an upper is getting the barrel nut torqued down to an appropriate setting (between 30-70 lb-ft) while lining up a hole in the nut so the gas tube can pass through to the receiver. It's very easy to have the armorer's wrench slip off and ruin a barrel nut. Ask me how I know.
I encourage everyone I know who's looking at AR's to build their own lower. It gives you a better understanding of how the gun works and it's really rewarding. Building an upper is probably not for most beginners, but if you're willing to invest in the tools and you're fairly mechanically competent it's not a big deal.
