As mentioned, the A model had a 250-C18, which wasn't cycle limited, and a 3000 lb MAUW. The II and III have a C20B engine, and the III has a 92 gallon tank, which has been subject to a 50-ft drop test with all that fuel in, which the ordinary 76 gallon one with a range extender hasn't.
Otherwise, there's not much real difference, aside from styling and looks inside, except that the III has a higher internal MAUW of 3350 lbs (as opposed to 3200) if the 65" tail rotor is fitted, together with a placard or two. I think the II can have the same mod, but I'm not sure.
Phil