People say buy this game just for the mods. Does that mean I should be getting them for the first playthrough of the campaigns?

'People' say a lot of things. First of all, you will completely miss what the game (any game) is all about if you don't give it at least one playthrough without modding it. Then, if you think that it could be enhanced by user content (and often this is the case), then go ahead and browse the mod databases. But I do urge you to give it at least one chance as it was designed and intended -- you have nothing to lose.

People who say that are often talking about modules you play instead of the original campaigns, rather than mods to the original campaigns.
Although if your talking NWN2, then the NWN2 facelift pack is worth a look. But it can be added or removed at any time without effecting your saved games.