Now the game is coming back to me again (I've been playing Wiz 8 again and the DD details faded).
What I did with all my characters was to take no more than a point in PP or LP and use items and the thieves guild to build up the points. Pickpocket is useful in only one instance, and that's optional (but worth doing).
My preferred character was a fighter with a few spells, but I did finish the game with a mage, who did complete almost all the quests (there were a couple I missed, but they weren't critical). So yes, the mage is a viable character.
Note: merchant inventories are randomized. The items for sale are chosen the moment you talk to the merchant. I always saved first, because (after I knew the game), I had a good idea of what I wanted. Stocks turn over at different rates for different merchants, with a minimum of 24 hours since your last visit. Most are longer than that.
You need either lockpick 3 or telekinesis 3 to get the door open (TK will move the lever on the wall). Just so you know for the future
Nightowl2