If you get the hand guard before the gas block, make sure you get a gas block that will fit under your hand guard (if your hand guard would be long enough to cover it) this would probably only be an issue if you got a heavy or bull barrel and a pretty slim hand guard. The hand guard specs should detail out the inner diameter for you, and the gas block specs will typically give you the outer diameter.
One other tip. while your torquing the barrel nut, make sure your torque wrench isn't secretly hitting the bottom of your upper vise. If it does it will drive you completely nuts trying to figure out why you cant get things lined up and the nut seems to tighten up all at once. lol.. I still get frustrated thinking about how annoying that was.