Building a web directory is not easy and you need to invest time and money on your directory in order to make it successful, as i understood you intending to build a niche directory which is good since niche directories are more focused on one subject and thus they are more easy to maintain, promote than general directories.
To make a successful directory you need your web directory to be unique than other directories in order to attract visitors and offer them something which would make them come to your directory rather than going to other directories.
Avoid mistakes and make sure that you gain enough knowledge about developing, maintaining and promoting a web directory before you start your project.