[Radek]

As far as a thief in your party is considered, you must have some kind of thief with you or you will suffer to the end of the game. The game is overtrapped and only a thief is able of removing traps. A monk can detect traps, a mage can also detect traps by means of a spell, but only a thief can remove traps.
Therefore, a thief is a necessity. What kind of thief - this depends on your play style. If you are a "fighter" or a "mage" and if you need a thief only for detecting and removing traps then choose a dual class thief/something, or, perhaps better, a swashbuckler/something. Invest all your skill points into Detect Traps and Remove Traps and dual immediately. If you want to specialize in thief abilities like backstabbing or using poisoned weapons then select a corresponding type of thief.
If your protagonist is a dual class thief/something then you get a thief stronghold most probably. Also, you will not able to work for Bodhi. If you do not want that then you must not have a thief protagonist and you must accept some predefined thief in the game. It's a problem because the offer isn't good:
Imoen - you will lose her after leaving Irenicus dungeon and regain her late in the game.
Yoshimo - you will lose him in Asylum.
Ian - the only thief who will not leave you.
Therefore Ian. The only alternatives are a thief protagonist or a custom party.