If you haven't played any of the IWD series before, you will find that it is
very linear and battle-oriented compared to BG. Be warned.
Some good characters are the following:
Paladin; Druid or Fighter/Druid (I prefer multiclass); Cleric, Ranger/Cleric or Fighter/Cleric (dual at lvl 7 or 9, or just take multiclass); Fighter/Thief (multi) or Thief/Mage (preferably multi); Mage; and/or Bard.
Absolute necessities are:
1) 1 main tank - best armour, best shield, best AC - paladin or fighter is best
2) 1 or 2 (or more) supplemental tanks - I use multiclass fighters here. May serve double duty as healer (Fighter/Cleric or /Druid), mage (Fighter/Mage) or thief {Fighter/Thief).
3) Healer
4) Mage
5) Thief - not strictly necessary, but worth taking in some form just to avoid the aggravation at certain areas.
Double up and mix classes as you wish. I like having both a cleric and a druid in my parties, although not necessarily single-classed. Two mages (or specialists) are also nice to have (or mage and bard), provided that they can also serve other functions (as in thief/mage or fighter/mage).
Another note - missile weapons are devastatingly powerful in this game. Make sure every character is proficient or better in one, and make sure you have a good mix between longbows, crossbows, and slings. Any fighter combination class is capable of serving as a great "archer" unit in lieu of tanking or spellcasting.