so if I understand you correctly, you want to dynamically select the equipment.xml that you load?
easy to do with Qdabra's InfoPath Accelerator Web Service (DBXL) which allows you to pull XML into your form based on field values. you could have a dropdown that populates with the documents and add a rule that fires when one is selected. the rule has an action that queries the web service specifying the correct value.
SharePoint has web services, too, but I don't think they return the schema when queried.
the other option would be to use merge forms and have your team manually select the form to import. you have to add some fields to your main data source that receive the data from the merged form and then you have to define the merge xslt and xsd in the manifest - ImportFiles parameter. but, you should be able to do that relatively simply without code and without any third party apps.
you can purchase an hour or two of consulting time if you'd like one of our MVPs to show you the magic. :)