In basic HOW, there's only two things you need from your characters: (1) fight and (2) heal. Everything else is for aethetics...

<font color = mediumspringgreen>Well, theoretically, I guess NO class is imperative. However, I would slip Theif skills in between bashing and healing as the most crucial to a non-frustrating game.

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What's a party,
without a song?
Bards ROCK!
Party On!!
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MILD SPOILERS.....

O.k., being able to find and remove traps is "nice". I might take a human thief to level 2 or 3 (putting all thief skill points in find/remove traps), and then dual to another class. With an 18 Dex, Find/Remove Traps would be at 60 or 80, and there are plenty of potions and items that can help improve it further. Maybe: Thief 3/Cleric 30. That takes care of both traps and healing.

Too bad there isn't a Ranger/Thief option available. That would be a really good character.

Thieves are so easy and cheap to level up, though. Why wouldn't you let them level a bit? If you went up to lvl 7 or so (a relative pittance of XP), you could max out their stealth as well. Just a thought.