Aaron,
Yep, basically if you want a accurate E in 1/72 scale add the winch and rotor brake to either the italeri B or C kit with a 540 rotor for the C. You could also steal a 540 rotor from a 1/72 cobra kit as long as it had the sqare-tipped old style blades and was from a model with 44ft rotors.
In 1/48, you could either get the UH-1C update set from Cobra Company and do a C-based Echo or just get a UH-1D/H rotor and cut 2 scale feet off each blade and add blade counterweights. The Huey Hog is basically a B with a 540 rotor. Hey, you could build the infamous 540 B straight from the box! The only isssue with making a B-based acho other than the rotor would be the late style particle filter. All B-based Echos I have seen had the bell mouth intake instead. You could graft the bell mouth intake from the older Monogram/Fijimi UH-1B kit. That's really about all those kits are good for anyway. Oh and whatever you build remove the nose mounted FM antennas from the nose as no Marine Echo I've seen had em. For a B-based Echo don't forget to leave off the roof mounted pitot and make a nose mounted one as well.
As far as armament, you really need the TK-2 kit used by the Marines to be accurate. currently, that is unavailable in any scale that I am aware of. Also, as far as I know no Marine Huey ever carried the M-5 40mm grenade launcher on the nose. The TAT 101 turret would look cool though, but alas there is no plastic available for this either to my knowledge. If there was any chance that the Marines ever flew with Army weapons, it would be the 20 UH-1B's we've been discussing above. Never seen a pic, so can't really comment there, but even then they probably had Marine weapons if they were not configured as slicks
UH-1E slicks are perfectly legit for Vietnam as you can clearly see in the book link posted by Supercobra above.
For UH-1L, TH-1L and HH-1K info go here (read beyond the initial rotorhead discussion):
/forums/785871/ShowPost.aspx
For more on the Echo go here:
/forums/804031/ShowPost.aspx
Hope that helps,
Ray