All of this is covered in MANY threads. a quick search for "handguard" yielded 360 results Smith & Wesson Forum - Search Results

To answer your questions:
A) Yes, you need an AR-15 barrel nut. You will also need a handguard end cap, 10-32 screw and a gas block. Get a gas block with a rail-height picatinney rail and you can mount your flip up sight on it. Your gas block will probably have a .750" inside diameter for a standard AR barrel and you will need a short length of brass tubing, slit lengthwise, to use as a spacer.
B) Use a rail height gas block. It will be easiest if it is a clamp-on model.
C) I have never used a barrel shroud. Keep waiting for a response or go through the picture thread and see which one you like.
D) The "ears" to remove are actually part of the receiver, not the barrel. They positively locate the OEM handguard and keep it from twisting when the gun is assembled and the factory barrel nut is tightened.
Here are pictures of my conversion: