[Ode to a Grasshopper]

If you have both expansions you can get up to level 40, if not then I heartily recommend getting them as the original NWN single player campaign is pretty disappointing, whereas the expansions are much better and usually necessary for the better downloadable fan-made modules. Henchman inventory control in SoU and HotU and having 2 henchmen in HotU is another nice addition. I think you need them for the Community Expansion pack as well.

If you're going with RDD I'd suggest a few combat-class levels (barbarian is good for the 10% speed increase/weapons proficiencies/d12 HP increase and you don't need the heavy armor proficiency, take at least 2 levels for Uncanny Dodge I). RDD mostly boosts AC, HP and STR, and so isn't so useful for 'pure' spellcasters as those with some melee abilities. There should be some light/medium armors that have arcane failure reductions too (again it's better to have the expansion), though they're still not so useful as some of the better mage robes.

Ultimately it comes down to whether you want your character to be a spellcaster who isn't quite so vulnerable/ineffective in melee as 'pure' spellcasters (take a level or 2 of barbarian or whatever combat class takes your fancy), or a melee warrior with some magical augmentation/offensive spells (in which case buffing and/or defensive spells are your best bets with one or two area-effect offensive spells). Another advantage of buffing spells for the latter option is they can often be used on both henchmen and familiars, and if you only have a few Sorceror levels your familiars will be pretty weak.