Typically the 855 is surrounded by other messages (850 PO, 856 ASN, 860 PO Change, 865 PO Change Ack). Are any of those required, or is this purely the PO Acknowledgement message? My delivery would be dependent on the specific database technology (e.g. SQL Server, Oracle, DB2), as newer versions of SQL Server provide XML capabilities directly in the query syntax. Also, there are multiple XML schemas available, is there one in particular or will the specific XSD be provided?
If a simple XML message is desired, based on the assumptions I am making the generation from a SQL table should be fairly straightforward.