Hi - with repeating data & promoted properties, merge is about the only option that is going to include everything. You could put your attachment field into the repeating table for the other information on the line item, by the way - it isn't clear in the documentation, but your attachment field can be repeating, or can be a field in a repeating group.
I'm little confused about the workflow part?
EDITED TO ADD: Instead of just saving the attachments to a library, you could write the repeating data to its own list, and include the attachments (if the attachments get moved into the repeating group) as list attachments. That would also return the URL to the form for the attachments, and you could run workflow off the list - just another idea, since I'm not quite sure what you are trying to do. :)