FYI the lower receiver IS the core of the gun. It is the part that is sold LIKE a gun, NICS check - sent through an FFL etc. The rest of the gun is your call. Buying an 80% complete lower receiver avoids the FFL and NICS check. It is essentially an unfinished part. In this case a hunk of fiber reinforced cast resin. You can buy them in alloy as well.
If you are thinking of trying an AR - look at the DPMS Sportical and Oracle. they are solid guns by a trusted company at a very good price point.
If you want to take on a build I might buy a stripped but complete lower and do the FFL etc. rather than take the 80% route. There are MANY good guides on AR builds on the web. The guns systems are very modular and straight forward — IMHO.