Magento: send a custom ordermail with a xml attachment

I have to make a Magento plugin that sends a copy of a successful order to a business partner.

This email should have an XML-attachment with all orderdata + customerdata.

I can create a simple send function like http://ift.tt/1vdXkQC.

But where should I call this function when a order is finished? And how can I create an XML-file with all data?

Thanks!