Welcome to the forum. Lot's of good information here, as you already know. Also, lots of help as well.
1. I'm a novice builder too, almost done building my first. It's a little daunting at first, but it's really fun actually. You won't have to do your own smithing to be honest.
2. Entry level ARs can be just as fun to build, and just as fun to shoot.
3. Building vs Buying is debatable. In my experience, most people are building to get exactly what they want the first time around. Buying is also great too if you're on a budget and can find what you're looking for. Personally I built my first one because I didn't want a bunch of parts laying around, and I wanted to pick and choose to my tastes.
4. If you're interested in what you need for a build list, you'll need to do some home work on what you want to use the rifle for. From that point you can begin to put together build list. You might also look into the links to read at the top of the "Build it yourself" section. Their is lots of good information there that could potentially answer your questions before you need to ask them.
5. If you want to shoot both 556 and 223, then you need to go with a 556 barrel. Iron sights and optics can both be on one rifle at the same time if that is what you prefer.
6. If you're looking to stick to a budget, you might want to consider looking allocating funds for barrel, trigger, and BCG first. Some people might tell you differently, and my answer might change in 15 years after I've been doing this for a while. The links next, are just some of the more recent threads with regard to lower budgets. Really the trick is to be patient and wait until you can get a steal on the various sites.
http://www.ar15.com/forums/t_3_4/651391_New_Budget_Truck_Gun_Build___Opinions_are_welcome___.html
http://www.ar15.com/forums/t_3_4/651398__450_AR_build_.html
9. Barrel twist is going to vary depending on what you're wanting to do with the rifle, and what kind of ammo you're going to shoot. Again, certainly something you can dig around for here. The key is to try to avoid posting questions that have already been covered in depth. I personally don't mind, but some members of the internet goat locker seem to get a little bent out of shape about it.
Most important of all though, learn. Share the knowledge with others, and have a good time doing it.
EDIT: Looks like some friends beat me too most of what I said, ha!