Do not assume for a minute that the 100-4 Engine is any lighter than the later 6's. The 100 engine weighs 700lbs dry which is more than most (if not all) V-8's. Why you may ask? The whole dang thing is cast iron...Rick--
I have never driven a BN2 but apart from the better transmission and a couple of other refinements it is essentially the same as a BN1. The 100's are somewhat different in character from the later Healeys, and because of the lighter engine lump have easier steering and better handling. They are harder to find since less of them were produced and they are of course that much older than later six-cylinder cars.
If you can get your hands on a decent 100 go for it and count yourself lucky--I love mine.
Dan M.