I would recommend that you set up a Custom Object that mirrors the form data that isn't captured elsewhere (on the contact record, for example) and add a processing step to your form to Update Custom Data Object - With Form Data (perform appropriate mappings, etc). You can create merge fields from the custom object records and add these to your email.
Instead of using the processing step Send Submitter an Email, use Add to Campaign, which can include a wait step of a minute or so before the email is sent. This will allow the process to get the data into the Custom Object before the email is sent.
Test, test, test before launching.
Hopefully that will get you started!
Was thinking the same