It depends on what "look" you have in mind. If you want to put together something that looks like what the mil is using then get M4 barrel. If not, the plain barrel. Some of the "plain" barrels are what they call Hbar, heavy barrel, and they weigh a bit more than the M4 style barrel. There are also lightweight "pencil" barrels.
There are kits to attach rails to standard handguards but IMO they are best avoided. They are not "solid".