Have you seen the prices that Oracle asks nowadays for their licenses? MS-SQL is a lot cheaper. PostgreSQL is open source, is actually quite similar to Oracle and suits 1000-people companies just fine. DBA skills in any of the mentioned databases so far will be the main decision making point.
The Xerces (C++) library is very good (open source) and extensive with XML which will take a lot of programming work out of the equation. XML/SOAP is the combination of choice when letting 3rd party apps communicate with something like SAP. Getting your head around creating proper XML communication will be the next problem.