You can call the main webassist number and talk to someone in the sales department if you have further questions. You will probably want to purchase Dreamweaver if you aren't using it, and a copy of the Web Develper Suite. Then install all of the extensions in the Suite.
ECart may be the only one you need for now, but who knows what else you may need to update if he used so many.
Once you purchase the suite, post a support incident with your error and page it occurs on and somebody will be able to help you debug the problem.
All of the development files are stored online, so if you wanted to and are familiar with the server languge you can just download and start debugging the problem manually.