Goal-Directed/Interaction design method for website

Half way through final UNI year Computing project:
Building a dynamic website with CMS (and possibly stock control) for a tyre company where products can be purchased online.

  1. Given that I am using Joomla and Virtuemart, is it worth using Goal-Directed design? I mean can I customize Joomla and Virtuemart enough to warrant this methodology that involves iterative design stages? Can I use personas with this kind of pre-written software also?

  2. What is the best/most common design methodology for this type of website given the pre-written software being used?

  3. Maybe just use some of Goal-Directed design ideas like iteration - should this be done?

    Thanks anyone