I've got a few things to point out...
first, If you want to do it, and are willing to work at it, do it. Everyone needs a project, right? If you decide to go for it, just keep your expectations realistic. IF you do manage to get it done well enough to fit everything together, it still may not be terribly accurate or reliable. Even some of the companies who have invested a ton of $$$ in tooling and machining, still can't turn out quality AR's. It is a given that it would be a LOT cheaper to simply buy a lower reciever and a set of parts and build it yourself, but from the sound of it, that just may not be satisfying enough for you. Like I sadi, if you have the time, money, patience, and desire,...go for it.
As for the legality of building your own firearm, first, don't take what ANYONE here says for fact. Do the research and make sure you know what you're getting yourself into. Check all the laws that may apply - Federal, State, City, County, etc. The BATF web site is a pretty good source of info on the federal side, and to gat you started, I believe it says (again, don't take this as fact, research it first) an individual can legally manufacture a firearm as long as said firearm is for personal use and is not to be sold, traded exported, etc, and as long as it's construction is in compliance with all other federal and state laws (must be built as a POST BAN, so no evil features). Thats just from memory, I've never really wanted to build one from scratch, so to speak, so I haven't looked all that hard into it. Hope this helps, or at least encourages you to go for it, as long as you count the cost and understand how much work is involved. Good luck.
-John