re: AR-15 for dummies.

The lower receiver is the part that houses the trigger assembly, safety, accepts the magazine, and accepts the buffer tube and grip. It also carries the weapon's serial number and must be transferred to you by a FFL holder. The only part that has to be transferred is literally the receiver itself. Everything that goes in or attaches to the lower receiver can be shipped to your front door and can be assembled with a couple of punches, though there are a couple of parts (the retention and pivot pin assembly, specifically) that could use a specialty tool. That tool is a 1/4" clevis pin and should cost you about a buck at the hardware store. A vise would also be nice to be able to hold the receiver steady while assembling.

The upper receiver accepts the barrel, gas system, bolt carrier group and charging handle, the dust cover over the ejection port, and the forward assist, if you have one. The barrel and possibly handguard will require some grunt to get them installed and WILL require a vise to hold the upper, an armorer's wrench to turn the barrel nut, and a torque wrench to attach to the armorer's wrench that you can dial in and input the grunt into the barrel nut. It wants more than 30 ft-lb, but less than 80 or so, and the torque wrench is a decent idea to get it into the range. You tighten to 30 ft-lb, unscrew, tighten again, unscrew again, then torque to your final torque in 5 ft-lb increments. The rest of the stuff in the upper can be put together with hand tools and punches.

It is by no means impossible to mount a barrel at home, it's just that it requires a dedicated bench, vise, space to turn a torque wrench, etc. It's generally a good idea to mount a reaction rod or a vise jig to transmit the torque to the vise without passing it through the body of the receiver to keep from twisting it, and that's another specialty tool you'll need to mount a barrel. Since it takes so much more room and tools to mount a barrel, I generally just opt for completed uppers and put the rest together myself. The completed upper generally comes with everything mounted, but with no bolt carrier assembly or charging handle. I recommend that if you're interested in building, you mount at least one barrel yourself just so you know how. If you're not interested in learning how to build one up, you could just buy a complete rifle instead.

Of note is that if you buy a complete rifle, you pay the 11% (I think) federal excise tax that gets applied to completed rifles. Taxable items contain a lock/action, stock, and barrel. This tax is paid one time and paying this tax is the responsibility of the maker of the item the first time the title to the item changes hands after it is created. So, the cost of the 11% tax is built into the sale price, so you never really see it as a line item. But it's there, passed on from maker to distributor to FFL to you. The maker of a rifle and the manufacturer (might not be the EXACT terms used by Treasury and these terms might not mean the same thing when the ATF/DOJ uses them that it does when Treasury uses them) are not necessarily the same thing for tax purposes, however. The manufacturer is the entity that manufactures the parts, but the maker is the entity that makes the firearm and produces a taxable item from those parts. When you buy all the parts from the manufacturer and assemble it, you are the maker of the firearm and are responsible for paying the excise tax upon transfer, not the company that manufactures the parts. So, when you buy the parts, you are not buying a rifle and the vendor just tacks on the normal sales tax on the cost.

Since the excise tax must be paid on business use, if the taxable item is used for personal use it is exempt from the excise tax. On a $1000 rifle, that's about $100 you're saving. It's worth noting that the TTB (left over in Treasury when the ATF got moved to DOJ) hates citizens as much as the ATF does, so their definition of a "complete rifle" is not the same as yours. If you buy all the parts at once most, if not all, vendors will attach the excise tax because the TTB will claim that merely putting parts in the same box constitutes creation of a taxable "completed rifle" that is being sold for a business use (the maker's profit) even though you have to spend hours working on it and assembling it before it can fire. Best to buy the lower (the stock part of the lock, stock, and barrel that are required for an item to be taxable) on a different day than the rest of the parts so they're never together until they get to your home. Or, better yet, buy the lower receiver from a different vendor than the rest of the parts. Then, you are producing it for your own personal use, and presto-chango, no excise tax. If you sell it, however, be aware that you may well be responsible for the excise tax if the feds ever want to put your pickle in a vise.

Of course, the FFL that does the transfer on the lower receiver is going to charge you for that service if you're not buying the weapon from them (unless the FFL is something like a range that you have a membership at that gives you free transfers) so it's likely going to be close to a wash on the all-in cost, but I thought I'd throw it out there.