Well, I haven't done much research on lighting in the 18" range, but based on the plants you've listed, I'll go with my initial recommendation. 24" down is a long way, and even though many of the plants you have are lower light plants, you'll probably need as much as you can get to the bottom.
I'm building a viv with the 24x18x24 exo-terra, and I'll probably be going with two 55W compact flourescent tubes, each providing about 4100 lumens. You might consider using the 36W versions; they give about 2500 lumens, and should last quite a while.
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