From what the experts say, Intellect 19 is all you need, even for a Mage. To get all of the best dialogue options (with few exceptions), Wisdom 23 or 24 should suffice. (25 is the max for every stat.) Charisma 16 helps in a couple of cases that spring to mind, but high Wisdom usually trumps everything else. There are other ways besides levelling up to get stat increases, by the way, so if you play your cards right, you'll get awesome stats by the end of the game.
You shouldn't worry about completing the game. If you follow the major storyline (which involves deciphering a few important clues), you will get to where you need to go. And if you play your cards right, you can avoid combat in most situations, so your characters don't NEED to be as tough as you might think. The main thing that might bother you about finishing the game is that you might miss some of the quests and dialogue (there are so many of them), and even if you finish ALL of the quests, you still won't get answers to all of your questions, and you'll just have to live with it.
I'll give you a couple of spoilers just to give you some comfort, okay? But if you don't want any spoilers, don't read them:
There are two areas in the game where you can earn unlimited experience. One is Under Sigil, where the monsters respawn endlessly and you can kill as many of them as you like if you don't mind repetitive battles. If you go there later in the game, you'll see some monsters that you didn't see before, and in rare cases, they will drop items of massive power than can improve the abilities of your party members.
You can also buy the Modron Cube in the Curiosity Shoppe. You should not pass it up. One of the joinable NPCs is hidden inside it, and in my opinion, he is indispensible. For one thing, he's the only NPC who can use missile weapons. With the right equipment (such as the Lens of Seeing Double) he can kill most monsters before they ever reach you. The Modron Cube also offers an endless supply of monsters and treasure, and it can enable you to reach insanely high levels if you don't mind repetitive battles. You can also use the cube as a teleportation device. I'm not sure if it was meant to be used that way, but it worked for me.
There is also a trick you can use in the Alley of Dangerous Angles to get thousands of gold pieces in treasure. Just lure gang members into the opposing gang's territory, let them kill each other, and then pick up the loot. It isn't possible to do that anymore if either of the gang leaders is killed, so I don't know if you have already missed your chance. I used the gold to buy tattoos at Fell's tattoo parlor, which is a great way to boost stats.
Good luck!
