Directory listing website for Niche... General thoughts and questions

I have done TONS of websites in WP, but this will be my FIRST using ONLY WPMU stuff, so I want to get it right and hear from others..
This is what I am doing. I am creating a Niche Dir Listings site that people will pay to have access to, but I want to be able to do it right. Monthly payment through paypal etc with ease of signup and having mulitple admins to add the directory listings..

Where to start? I know there is a DIR listing plugin, but wanted to see if any of you have done something similar I can bounce ideas off from.
Thanks a ton!
~ Jon