Hickory: Recharge your wand of fireballs. Take scrolls of Protection from Petrification or potions of Mirrored Eyes. Take potions of invisibility for Imoen (scouting) *and* your cleric (invisibility and Turn Undead == profit). Take cure poison potions. Take potions of Speed, but use them wisely. Give Minsc Spider's Bane. Anything else? Yeah, but you'll work it out...
Imoen and Dynaheir both have Wands of Fire. Dynaheir also has Protection from Petrification memorized. For some reason Imoen can't learn it? At least not from any of the three scrolls she has.
Minsc has Spider's Bane and Boots of Speed, so he's hasted and immune/highly resistant to Entangle and Web. Main has The Stupifier mace and a War Hammer +1. Jaheira has a sling +1 and a scimitar +1. We do have some oil of speed and some invisibility potions, so we should be okay there.
NotJabba: The challenges in Durlag's Tower are extremely varied, so I can't think of any particular advice to give you that will hold true for the whole dungeon. I played using the same party you're using (except playing as a Monk instead of a Ranger), and I found the most useful spells to be the same ones that are most useful everywhere else, particularly Haste and other buffs. If you've explored most of the areas in the game, you're probably as well equipped as you can be -- personally, I'd go through the few other areas you haven't played yet just to be sure, but it probably isn't necessary. You really should hit up Ulgoth's Beard, as I recall the shop there had some pretty good magic equipment. I don't remember exactly how to reach it, but it shouldn't be too difficult -- probably you need to head west off of the portion of the map just outside of the entrance to Baldur's Gate.
EDIT: Good advice from Hickory, who I'm sure remembers more of the specific monsters than I do. One thing I'd note is that you might just want to have Protection from Petrification memorized instead of using the potions of Mirrored Eyes -- there are only enough of those potions in the entire game for one use on each of your party members, iirc, and you'll probably want to save them for *after* Durlag's Tower. However, they might be more abundant in EE.
Apparently not too many as it appears I only have two potions of mirrored eyes. Lots of healing potions though. I haven't used many potions at all during my playthrough, so Jaheira's been holding on to them in her potion case. =)
crumb24: SPOILER
Go to Ulgoth's Beard before Durlag's Tower. There are two wardstones for the tower there. One is for sale by a guy in the tavern/inn if you pay off his bar tab, and another vendor is selling his entire inventory which includes a real wardstone and a fake one too. You can also set up a tour there of the Tower if you wish.
Just strolled into Ulgoth's Beard for a quick look around and maybe to sell off some things. I also need to restock my missile weapons – may have to go to Beregost for that. Depends on what’s for sale here. I did run into a smuggler outside of BG. Refused his services. Hope that wasn't a mistake, heh. The intro text to the town did mention the tour, so maybe I’ll check that out as well.
kmonster: Do the exterior and the upper levels of Durlag's Tower first. Then travel and finish the other areas before coming back for the lower levels.
I’ll keep that in mind. There aren’t many other areas for me to complete, but I did open up another area north of the Baldur’s Gate bridge, so I’ll check that out after Ulgoth’s Beard. Wasn’t there an island you could access from UB as well?