About 6 or 7 years ago I ran into a local company that was doing something similar. The guy set up a business portal site and was charging businesses $100 per year for setting up and maintaining a basic 3 page web site. Beyond that, he charged for additional services. He wanted to offer a $500 per year SEO option that would guarantee that a businesses' 3 page site would rank #1 in Google. I declined to participate in that. At the time he had almost a thousand clients.
Today we would call a site like this a local business directory.
I have never seen any of the guy's portal sites show up in search results.
I think this could be done successfully if you maintained separate sites for each industry so that each site has a specific industry focus and provides enough combined content focused on a single keyword theme. Trying to develop a successful local business directory can be very time consuming and expensive. Also, given the nature of successful local business directories, it would be hard to charge a fee for participation in the directory when the most popular directories are free.
Andy, my gut feeling is that your idea might work for mid to large sized businesses, but small businesses have been scammed too often by Internet advertisers and they are hesitant to spend money. In the USA, you might be able to get a small business to pay $100 to $200 per year. Larger businesses are much more likely to pay the $997 setup fee and a monthly fee if you can demonstrate top 10 rankings.
Just my humble opinion.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin