That's why I was asking if just displaying the rich text would be adequate for your needs - otherwise you'll need code or qRules to copy the rich text from your SharePoint data source into your InfoPath form data source.
As to the second part of your question, if you search for cascading dropdown on this forum or in a search engine online you should find some good information - here's one example. Basically, if your module is a dropdown control, you can add a filter to it to only show the modules for the selected brand. If this is a browser form for SharePoint 2007, however, you can't use filters on the dropdown - here's an example of another approach.