Once a selection has been made in the dropdown, all you need are rules (Set a field's value) to populate your other fields. When calling these actions, you'll need to filter. After all, you have the entire set of list data in the secondary data source (from SharePoint) but you only want the data for the specific item selected in the dropdown. For info on filtering in InfoPath, here are a couple of articles and here is a previous thread. These may not be exact to your scenario, but a Google search on filtering from a SharePoint list might give you some additional tutorials. Good luck!