If you go heavy on Rokugan ninja, the ability to use the dodge feat against multiple enemies can be exploited. The elusive target feat (Complete Warrior) negates bonus damage from power attack dealt by your dodge target.
A ring of the darkhidden (Magic Item Compendium) makes you invisible to darkvision for 2,000 gp. If you're mostly going to be hanging around the underdark, that's almost as good as normal invisibility. Remember that you can't target spells against something unless you can see or touch it. A proper spellcaster will have a way to see invisible creatures, but creatures with spell-like abilities might not.
If you're going to attract your DM's attention to that item, you'll probably want a way to overcome it yourself. A hathran mask of true seeing (Unapproachable East) gives you continuous true seeing for 75,000 gp. That also happens to deal with most forms of concealment, which would otherwise prevent you from using sneak attack.
Some other items to look at are gloves of the balanced hand (Magic Item Compendium) and a gemstone of fortification (Draconomicon, p83).
