+1 on building your own. I'm saving approximately $300 by doing it myself. Its SUPER easy, fun and something you can say "yeah, I built it myself"
As for the legalities of the the build it yourself route, so long as you got the lower legally and follow all the NFA type rules wrt barrel length and such you should ne fine. the only thing you may have a hard time selling is ones that you buy the receiver 80% finished and finish the machining on yourself. it hink that puts you in a different category. Buying a complete receiver as you would any other gun means you can sell, trade... what not it later with no issues.