I have built a few ARs now including a few 80% lowers. There are only two specialized tools that you absolutely MUST have:
An upper vise block:
An AR combo wrench:
I have both of those pictured above. They are not the only tools out there, but they certainly work well.
Without the upper vise block you risk damaging the upper when installing the barrel, gas block and muzzle device.
The combo wrench has the necessary pins for the barrel nut and notches for the buffer tube nut. It also has a hole for a torque wrench.
You'll notice that I didn't include a torque wrench as a tool you must have. The barrel can be installed without the torque wrench, but it's risky. Generally, the barrel nut is installed until snug. Then it is torqued until the first possible hole is lined up to allow the gas tube to fit through. Most of the time, this first hole will be the right one. Even if you're using a torque wrench, going to the next hole will be too tight and using the previous hole will be obviously too loose. So, just tightening it to the first hole past snug should get the right torque even without a torque wrench. Even so, using a torque wrench is better.
I'm going to spare you my rant about torque wrenches. Suffice it to say that I have 30 years experience in calibration of measurement equipment and I recommend this combo of tools:
http://www.brownells.com/gunsmith-to...prod56974.aspx
There will likely be several who will say that a click type torque wrench is better than a beam type. For this application, they are wrong. Get the one in the link or one from a different source, I don't care, but get a beam type. It's the right tool for this job.
Then watch the build videos on Brownells. They are excellent videos and cover every possible aspect of the build.