What sub do you have now? I ask because it may help us give better advice. I don't think you can realistically tune an 18" driver below 20hz in 200L, with a port, without getting into some serious design issues.
PR's maybe but that adds a lot to your total cost. Sealed 18" has a much better shot in 200L.
For $600 you may want to look at the Rhythmik servo 15" kit, or maybe even the dual 12" version. They aren't known for wallcracking out-put, but they go DEEP and are said to have great SQ. Illka tested the 12" version in a tiny little sealed box and it did quite well. The 15" should be easily more powerful.