For HoF mode, you want to jettison your direct-damage spells in favor of stun/instakill spells because monster hit points are just too high for direct-damage spells to matter. So you're looking at chromatic orb at low levels, then gradually transitioning to emotion (hopelessness) and eventually wail. You then augment that with defensive spells like mirror image and stoneskin so that, in case your stuns wear off, you can absorb a blow or two while recasting them.
Even if you keep playing on Normal difficulty mode, stun/instakill spells are useful. Also keep in mind that a fair number of IWD's area-of-effect spells don't affect party members, so you can cast things like emotion (hopelessness) and wail in range of your melee characters.
What you don't need to do, on any difficulty level, is worry about the "strategic" spells you might be used to casting in BG2, from breach to secret word to true seeing. IWD is basically about bashing monsters over the head until they stop moving; there's essentially none of the complex mage-on-mage combat for which BG2 is renowned.
Solid tips. It's also worth pointing out that thieves receive stat points faster than everyone else (and much faster than mages) because their XP table lets them level up so quickly, potentially giving you about 10 extra stat points by the end of the game.
A large proportion of enemies in IWD are slash-resistant undead, so I'd keep that in mind when choosing weapon proficiencies. Flails especially are a solid choice due to the large number of decent magical flails in the game.
If you want to mix-and-match your specializations, you can switch between fighter and mage as many times as you want, as long as you reach level 7 as a fighter before you reach level 7 as a mage (and then reach level 12 as a mage before you reach level 12 as a fighter).
If you struggle a bit with melee combat in PST, or if you want to start the game with an 18 strength and be catapulted directly to 19, then it can make sense to get your first specialization bonus as a fighter even if you're planning to be a mage or thief for the rest of the game. Otherwise, there's something to be said for getting both specialization bonuses in the same class because you receive extra benefits for doing so.
For your first specialization bonus (at 7th level), you get +1 to STR/INT/DEX (depending on your class) and if you are a fighter, you also gain the ability to put one additional proficiency point into a weapon. For your second specialization bonus (at 12th level), you get the same thing. So for example, if you first reach level 7 as a fighter and first reach level 12 as a mage, then your total specialization bonus would be +1 STR, +1 INT, and the ability to put one extra proficiency point into a weapon.
You also receive extra rewards if you receive both specialization bonuses from the same class, which the game calls "double specialization":
Double specialization as fighter: +1 CON, +3 hit points Double specialization as mage: +1 INT, +1 WIS, and +5 to your lore score Double specialization as thief: +1 DEX and +1 to your luck score
So if you double specialize as a mage, you receive a total of 4 stat points: 1 INT for your 7th level specialization, 1 INT for your 12th level specialization, and 1 INT/WIS for having gotten both of your specializations in the same class. Whereas if you mix-and-match fighter and mage, you only receive 2 stat points.
Additionally, there are a couple of places in the game where you receive a small in-game benefit from double specialization. Two that come to mind are Fell's (where an extra tattoo becomes available) and the Pillar of Skulls (where one of your options for receiving an answer from the Pillar becomes less costly).