Some input on Business Directory / Classifieds on DNN:
The DNN Core Repository has a "Business Card" skin you can use when creating a business directory. I have used it for such a directory of businesses in a town in south america here:
http://www.santacruzperu.com/Contacts/HuarazDirectory/tabid/62/Default.aspx
The Repository module is very flexible with templating, customizable user upload/updates, ratings, comments etc. it also has a companion dashboard module with various designs (to summarize / show lastest, I use one on the homepage). I also use this module on the DotNetnuke websites directory that can be found here:
http://www.faktnet.com/DotNetNuke/DotNetNukeWebsitesDirectory/tabid/140/Default.aspx
The Core Repository Project has regular updates and the teamleader Steve Fabian & co are doing a great job with this module ( a new version should be out in a week or two). Here is the Project homepage:
http://www.dotnetnuke.com/Default.aspx?tabid=830
Other Ideas / options:
Biforce has a free Google Maps module with a directory listings feature, can be seen here:
http://dnn.bi4ce.com/Modules/GoMap/GoMapDirectory/tabid/288/Default.aspx
DNNJungle has a module (commercial for sale at snowcovered) called InfoMap, it can be used to created hierarchical InfoMaps showing ie. the businesses on a map with different icons for different types of business. This module is now using a provider interface, making it easy to write your own dataprovider. The module can be used for a lot of other use cases also some are demonstrated at the developers website:
http://dnnjungle.vmasanas.net/MyModules/Modules/InfoMap/tabid/35/Default.aspx
If you need hierarchical categories there is a developer called DotNetJedi that has commercial modules for Business Directory and Classifieds Directory. I have not used these modules, they can be found here:
http://www.dotnetjedi.com/DesktopDefault.aspx
There are certainly other options (besides writing from scratch), welcome to the weird and wonderful world of DotNetNuke!