Hard isn't all that hard, it's actually a cakewalk if you've beaten KH1 on hard. It'a been years since I played the game though so I can't go into great detail with advice, but I chose warrior and pretty much breezed through it. Also, don't forget to make good use of the RC system.
Born Jan. 6, '94 at 3:47 pm Gaming since Jan. 6, '94 at 3:48 pm
The difficulty is fairly well scaled, so choose whichever you like — Beginner, Standard, or Proud — because probably you'll forget the choice very quickly. All three present a Battle Report at The End.
One thing else hinges on the dream weapon choice, a brief extra movie prior to the Battle Report. Can't get the extra on Beginner, and must fill out the Journal completely on Standard.
FWIW, many have played on Proud their first playthrough, yawn-yawn, no sweat. Many have played on Standard first, too, and then grumbled over the Journal completion. So if you play games repeatedly, I'd recommend Beginner first, to get the story and a feel for the differences, then leave Proud for your second time around. Beating KH2 on Proud isn't worth crowing over with your friends.
KH2's battling is very different from KH, BTW, so your skill with KH may not prove too helpful a comparison.
On the other issue, the choice really only determines how Sora's level-up abilities are dished out. And that order doesn't adhere to the warrior, guardian, or mystic designations as they did in KH.
KH2 has two one-time stat boosts as well: A one-point boost to strength, defense, or magic; and an AP boost, from 0 to 2 points. If you peek into ElectroSpecter's walkthrough, BTW, read thoroughly because the actual requirement for AP +2 (1,200 munny) is reported belatedly.
No time of day stuff, BTW. No missables to the game either, save a few chests with ordinary items that're found during the prologue. Conserve your mythril synthesis items; you can get away with synthesizing "one of everything" with the mythril materials handed out (getting more is possible, but unnecessary with care).
Have fun with your run, whatever your choices.
Guides and other contributions: http://www.esque.com/slr/gamefaqs/ FFX, FFXII, KH, KH Re:CoM, KH2, Okage: Shadow King, Secret Agent Clank
To provide advice to a slightly different type of gamer then falcon did:
If you are only going to play the game once, as opposed to once for story, once for gameplay, and you want a decent challenge, then playing on proud is for you. If that isn't you, then feel free to do exactly as falcon recommended.