The Hasegawa kits were actually 1/450 scale (or thereabouts - the actual scale seems to vary slightly, e.g Vanguard is about 1/430). The range definitely included the Bismarck/Tirpitz, Yamato/Musashi, Missouri (probably issued as other Iowa-class ships), Akagi, and HMS Vanguard (this one was also issued by FROG in the UK). I think they also produced a Shinano.
According to Rajen's List (
http://www.quuxuum.org/rajens_list/shiprevs.html) the Hasegawa Bismark/Tirpitz kits are wretchedly poor (and the Tirpitz is basically a reboxed Bismarck) so the Kangnam kits probably aren't based on these.
I have the Akagi, which, as you say, is an excellent kit. Other than the odd scale, I can't find much fault with it other than the moulded-solid undercarriage legs on the Zeros (incomprehensible since the B5N Kates have seperate undercarriage parts!) On the other hand, the Hasegawa Missouri kit (which I was recently given by a fellow modeller) is very basic (though it appears fairly accurate in outline) and is downright crude in some areas - the Seahawk floatplanes are particularly bad. I don't really recommend this kit to anyone, though I will attempt to build mine as it was a gift (once I find some suitable references for detailing it)!
I've seen a Musashi kit at a collector's fair and it didn't look too bad, definitely more finely detailed than the Missouri, the seaplanes and AA gun turrets for example looked fairly good. Not sure if there is any difference between this kit and the Yamato.
The HMS Vanguard kit is one I've been trying to find for a while. It usually fetches fairly high prices as it's the only plastic kit (in any scale) of this vessel, the last battleship of the Royal Navy. The scale is an almost perfect match for the post-war cruiser HMS Tiger produced in 1/420 scale by FROG. (and in fact FROG issued the Vanguard under their own label for a while, along with a number of other Hasegawa kits)