You forgot gas tube.
If your Front sights is not also the gas block, then you need the gas block. make sure the gas block is compatible with your barrel's gas port and cuts.
If you're handguard is free float, it'll usually come with its custom barrel nut.
If not, you will need to get your own barrel nut, and delta ring assembly.
Flash hider and comp aren't neccesary, but helpful to have. A $2 A2 Birdcage FH will do nicely.
Don't forget the rear sight. (Unless you're using a A1/A2 upper)
Necessary to Build TOOLS:
Pin Punch Set/hammer
Upper Receive Vise Block/Upper Insert
Armorer's Wrench
Bench mounted Vise.
Useful/But Not absolutely neccesary:
If installing A2 Front Site/Gas Block, Get yourself a Gas Block Install block.
Barrel Vise useful for installing timed muzzle breaks.