I have looked into DD, that was the first company I ever wanted to go with, but I couldn't find prices, and the prices people told me were pretty high. I have also looked into getting a Mojo, that would be awesome, although I can't seem to find them. I have no dealer close. T-lines are hard to make, I have never made a box, so I don't think I would make that as my first box, who knows though.
I think the box would be too small, I i got a 15" it would be facing up as well. The biggest box I can get with that is like 4.3^3 ft tuned to 35 hz, which would be perferct for a 15, a little to small for an 18.