Are you talking about a solo game or a game with several party members? If you're going to solo the game, I suggest that you play a Wild Mage. If you're playing with a party, I suggest that you create two of your own characters in a multiplayer game such as a Wild Mage and a Swashbuckler (you can save the game and copy it to your single-player saved game directory), and then add NPCs as you see fit.
If there is a class I want to play, I think it's almost as much fun to play an NPC as it is to play a main character. Unfortunately, none of the NPCs is an Assassin, Swashbuckler, Wild Mage, or any number of exotic builds. However, I can edit any of the NPCs with ShadowKeeper to give me exactly what I want. For example, I can make Imoen a Swashbuckler or a Wild Mage. Yoshimo makes an excellent 7th Level Kensai dualled to a Thief (I give him the Monk avatar). Minsc can be a Berserker, Barbarian, or Inquisitor. You get the idea.
When you import a TOTSC saved game into BG2, the character's experience is set to exactly 161,000 points. Your character's Hit Points might be recalculated, a process which appears to be bugged. I usually don't bother trying to import a character; I just create a new one. I usually tweak my characters with ShadowKeeper, anyway (with custom colors and paper dolls, for example, and I give my characters the stats I want instead of rolling for several hours to get the same result).
Besides, BG1 and BG2 are so different, I treat them as two separate games. If you play a dual-classed character, you won't be able to create a character who is optimal in both games. In BG1, you should dual at 6th or 7th level; in BG2, you should dual at 9th level or 13th level or something like that, which isn't practical in BG1. So again, I'd rather just create a new character in BG2 and pretend it's the same character I played in BG1. I don't have any problem pretending when I play fantasy-roleplaying games.
