I would browse the AR15 sites on the web, alot, more than alot and read and study all the nuances of the AR15 platoform before buying anything off the shelf.
www.m4carbine.com
AR15.Com
Both are filled with members who have extensive experience building and shooting all varients of the AR series rifle.
If I where you, or you where going to take my advice I would purchase a quality stripped lower. "Quality" being the key word. Stay away from Stag, Armalite, etc. Stick with LMT, Colt, RRA, CMMG just to name a few.
You can find stripped lowers anywhere, but that doesn't mean it's a quality lower. There is a difference between MIL-SPEC and COMMERCIAL rifles and their recievers and parts.
Build your lower with a quality lower parts kit from Brownells. You'll get to know the AR platform more than you will ever purchasing a complete rifle just because it looks good on the rack and has a decent sharpie pen price written on the tag.
There are videos over at Brownells on how to install all the lower components, but you don't necessarily need all the tools that they may show you in their videos.
I've assembled a few lowers just sitting on the floor of my house while watching television with just a hammer, some punch pins and a couple other tools that are not expensive to purchase if you don't already have them.