This is just my opinion but I don't think it's the best idea to build the first AR you ever shoot or own, you need the experience of shooting one that runs correctly. Spreading out the cost may make it seem cheaper, and ARs aren't really difficult to build especially if you've been around them but they don't come with instructions, it's a plastic bag with 100 parts in it.
If you are talking about buying a complete lower receiver and a complete upper receiver assembly and pinning the 2 together then sure, find a buddy that has done it before and shoots and go for it, learn from a friend.
There are many simple things you can learn by reading forums, so many things I don't know where to start. The problem I see is if you try to save money by buying the cheapest junk on the market and end up with one that doesn't work correctly it will be a bad experience and you sell it and never look at an AR again or you have to sell it to recoup some money and start all over.
As far as the .223/5.56 ammo just get one chambered for the 5.56 it shoots both without going into details here.
Research products and prices try to find a book about basic AR operation and maintenance.
if you have specific questions I'll try to help more.
Last edited by Constructor; 04-01-2010 at 20:50..