I played KH2 before KH1.
I advise you get KH1 anyone since you'll want to get it once you play KH2 anyway ...
Story - KH2 = KH1 ... both have good stories
level desgin - disney worlds->KH1, origanal worlds->KH2
Gameplay - KH2 is waay better, though it suffers a little from limiting the need for magic...
For KH3 i hope the battle system is an evolution of KH2 -- but i want the level design to improve and the story to focus a little more on KH and less of following the disney worlds story ...