I'm specialized in Longswords, every since Ch. 1, in fact
That's not a bad weapon at all, so definitely stick with it at this point. Always remember that a longsword can be treated as a two-handed weapon if you unequip your shield. Doing so will increase your strength bonus to damage. If defense is a non-issue (for instance, against a spellcaster) then go two-handed.
As for items that grant immunity to mind-affecting spells, the easiest to find is the "Golden Circlet" helmet. I found three or four of these over the course of my most recent NWN1 OC playthrough. I'm not sure where you can buy one, and most of the treasure is randomly generated so if you haven't found one yet you may be out of luck.
Also, sorry for putting all of my stuff in here, I didn't want to start a new thread for something new everytime.
I don't think anyone has an issue with it :-)
Mind affecting? -> Potions of clarity
If you know what's coming then you can always chug the potion before you encounter the offending effect, but that's not a reliable option for someone who doesn't know the game very well or if you need to use it for repeated encounters. For getting through Klauth that'll work just fine, for avoid stunning effects from those Slaadi not so much.