About the software, I'd usually go with the "use existing software" approach. However, with the specs outlined (game mechanics and so on) there's also a big risk on ending creating a big patch "hacking" the existing software.

A third option in between would be to use some kind of framework as Django (or Ruby) with as much already done apps for the functionality as possible. For example, a comparison page for forum appz solutions on Django: http://djangopackages.com/grids/g/forums/