i'll give you the short and dirty answers to your questions:
1. difference is the twist rate for the barrel. the bullet weight that you'll be using primarily will determine the rate of twist. heavier bullet = faster twist
2. personal preference
3. the ar15 is a gas operated system. the gas tube goes from the upper receiver to, rides along the top of the barrel (under the handguards) and end in the front sight. if you don't want a standard front sight (or none at all) you need the gas block to take the place of the front sight.
4. don't bother, unless you are building a 10-rd full auto.
hat6. free floating "increases accuracy" by having the handguard not touching the barrel. if your looking for a cqb type rifle, you don't need to free float because most of your shots will be at fairly close ranges. my work rifle is not free floated and i have no problem hitting what i aim at in shoot houses and at work.
all of the rest of the research is up to you. hope this helps to get you started.