Database Software

Our company is at the initial phases of replacing all our invoice storage from PDFs into XML Db. This is roughly 80,000 invoices created and stored monthly, printed immediatly with moderate web access for historical reference. The schema definition is subject to change periodically as new fields and requirements dictate additional information be contained on the invoice. Past data stored will never be updated, but again, queried based on 3 nodes (cutomer, date and invoice number) and also searched via full text for keywords predicated on 1, 2 or 3 of the same nodes.
The question is based on these requirements, would structured or unstructured storage be a better fit. I have read the appropriate documentation that compares the two approaches, and have not reached a clear decision. We already know that we will want to partition this data (by month) and that will require upgrading to 11g. Any recommendations with appropraite rationale would be appreciated.