It's rather difficult to brew a world-class Witbier using extract/steep techniques. Some of the crucial ingredients - oats and raw wheat - must be mashed, requiring at least a partial mash.
That said, Northern Brewer's Witbier kit is pretty good, as is Midwest's. Both can be improved by the addition of some Supah-Sekret Spicez; I like cardamom and paradise seed in addition to the coriander and bitter orange peel.
I don't recommend flavor or aroma hops in Witbier. They clash with the spices and yeast flavors, which defeats the purpose.