I would think that the Canadians (http://www.dartcatamaran.ca)are the best at modifying the Dart 18 to suit their needs, and the UK site (www.dart18.com) will tell you what is class legal etc.
UK prices are expensive.
At the moment I am looking at the Hobie 16 Worlds site where they are boasting of having 64 cats and close to 300 teams - I sailed at a Dart 18 Worlds where there were 301 teams and each one owned their own boat. 301 Dart 18s on a start line in 1991. Now that was something !