How hard is it to build an AR and how much will I save by assembling the gun using all Bushmaster parts over buying one new?  I think I would have fun building one but I wouldn't get the warranty and I don't think I would save that much over buying one just like it factory built.  Is this true?  Are there any parts in assembly that require a gunsmith?  I am also going to build a FAL but am going to build the easier of the two first.

If you are intending to build from 0-80% lowers there is quite a bit of work and care that needs to take place. Initial builds of 0-80% receivers can amount to quite a bit of money if you don't already have the tools to accomplish such a task. This is where "economy of scale" comes in to play; the more you build the more you save.

If you are intending to build your AR from kits, it is fairly straight forward. Of course there are other factors to consider:
1) Completely stripped kits obviously take more time and may require more tools. You may save a few bucks, but the amount of time you put in to it can either amount to entertainment or a chore.
2) Of course Money is the biggest sticking point in most things. I assume you want a fine rifle, therefore you want fine components for your AR. The more you add the more you pay... A 500 dollar build can easily climb to crazy levels.
3) The most dangerous of them all... The Wife. Publicly spending too much time and money on your AR will make her jealous, upset and speed up the onset of her woman problems (PMS). Secretly doing this will result in all of the above and suspicion that you are cheating on her... I learned the hard way.

With all that said, building is very fun and addictive. Do not assume that you will build just one.

Just finished my first lower. It was a piece of cake!  Of course, that was after reading two months of what these guys did right and wrong and followed the old AR15.com instructions.  No special tools unless you count my son which was my vice.