I would use Drupal, a free, open source content management system. It`s modular, so you start with a pretty lean core code and can add modules as you need to get the functionality you want. If the functionality you want doesn`t exist, it`s going to be cheaper and easier to write a module for Drupal to do that functionality than to write a solution from the ground up. I`m not sure you would need a custom module, though. You might be able to create what you want without programming using Drupal`s Content Construction Kit capability plus judicious use of the Views module.
If forums are your focus (and it didn`t sound like they were), and you are going to buy forum software, I wouldn`t go with the service suggested. The industrial strength forum of choice is vBulletin, which is frequently updated and has the further advantage of being widely used by a lot of savvy webmasters. There are a lot of hacks to vBulletin to get it to do tricks, and there might or might not be one that adds to vBulletin what you want. The problem with the vBulletin hacks versus the Drupal modules is that Drupal is built with an API to accept modules, and you are much less likely to need to rewrite your additions when the core platform gets updated.
The dDrupal site is at drupal.org.