So I can't help but notice that Neverwinter Nights: Shadows of Undrentide and Hordes of the Underdark add extra classes. Is there a way to disable DLC so I can remove the classes added for the original campaign?

If you are interested in playing the NWN1 OC as it was originally conceived, here is a compilation of all the changes inducted with the expansions and v1.69.

The scope consists of much more than than the prestige classes and may surprise you as to the challenge level which faced the owners of NWN upon its release in 2002.

To be honest, I should have finished 5-6 times the OC before getting the first EXPANSION (not DLC) and... it was easy at that time (in the hardest setting). Yeah, sometimes fighting the dragons you were lucky that certain elf cast Harm. But, fighter, cleric, wizard and Paladin didn't give too much problem.

And if the things were very bad... you could do the classical "Diablo" strategy (hit 3-4 times, teleport and coming back). But yeah, don't use Appraise (I thought was useless for the OC) and neither the Prestige Classes and you are facing the similar "easy" OC.

However, on my opinion the real experience for NWN were intended for the multiplayer. The OC was more a glorified tutorial of how making a module.