I'm assuming this is a keyword research question rather than a how to put keywords on a site?

Before you go spending money on seo services, you should at least know what you want to rank for. You can't measure your success if you don't have a set goal

The problem with making too broad of a directory is that you will be competing with massive directories. You could instead focus on creating a niche directory, something that you can actually rank subcategories for.