It's definitely worth the purchase. I'll prepay in full so I can enjoy it with reckless abandon on the day of release - I've toyed with importing FINAL MIX on a few occasions, and never had it in me to play 358/2 DAYS, so this would be a huge win, both for me and for Square Enix.
I'm afraid you're right though, 2.5 probably isn't far behind and will almost definitely predate KH3. I'd love to see that, but I'm still bummed that they seem to have a team working on this that could, in theory, be developing KH3 (though who knows - maybe they are and they just haven't announced anything yet...).
I just hope we're getting a full HD remake of these games here and not a plain 'ol upscale job. They're (supposedly) giving FINAL FANTASY X a full remake as well, so hopefully that means we'll see KH get the same treatment with brand new in-game assets from scratch, not just a glossy finish and redone cut scenes (which were beautiful to begin with).
Oh for sure! This is a great idea to reunify the fan base outside of Japan. Over there it seems to be way more common and acceptable to release series offshoots/sequels/tie-ins on different portable consoles, whereas here portable gaming can be pretty divisive: not everybody has time/inclination (myself included) to play portable games.