Tac,
I just built one up recently and it was a fun little project that I am sure you will enjoy too.
What I did was order a kit from Bushmaster (XM15 E2S Match 20" Target Rifle Kit.) which included everything needed with the exception of the stripped lower receiver. (The stripped lower must be purchased through an FFL dealer.)
There are some tools that you should buy to make the assembly go smoothly. First, get two special sets of punches. The first kind are roll pin starter punches. They have a hole in the end of each punch that "holds" the pin which makes getting it started in the hole much easier. Once the roll pin is started in the hole you will then want to use the other type of punch to drive it home. These punches have a small tit on the end which engages the hole in the roll pin. This punch will center itself and will stay centered as you tap it through the hole.
You will also want to buy an armorer's wrench which will allow you to torque down the receiver extension to the lower receiver. Another very worthwhile item to get is a pivot pin detent installation tool. Installing the pivot pin and its detent is the trickiest part of the whole assembly and this tool will make it a snap to install. Trust me, the $8.00 for the tool is a very smart investment.
All the tools I mentioned can be purchased from Brownell's.
http://www.brownells.com/Index.html
Last thing I would suggest is go to
http://www.ar15.com/ as mentioned by blooch and look through their website as they have much info available. Download the Army and Air Force M-16 technical manuals they have available. This will help you with your project greatly.
Most of all, take your time and have fun!
Feel free to e-mail me with any questions you might have.
Regards,
kgs.