Yes, that's a great deal for that Smith & Wesson. AR-15's are NOT difficult to put together. As I said, I put mine together with a laptop on my workbench, a hammer, some punches, a vice grip, and a how-to video playing on Youtube.
There are plenty of guys at AnandTech's forums who build their own PC's from the ground up. This is actually less difficult as you don't have to dink around with drivers, OS installation, hooking up case wires to the right prongs on the motherboard, etc.
Building your own evil black rifle is an awesome experience. When you are done building one and are sending hot lead down range at over 3,000 feet per second, you will have a huge grin on your face knowing that you have built this fire-breathing beast with your own hands.
When the Zompocalypse hits, will you trust your life to a firearm that some assembly line monkey put together at 4:30 on a Friday afternoon, or would you rather trust a rifle whose parts and inner workings you are intimately familiar with?