Ideally you'd want to change the column in the database, but given your previous comments, it looks like this isn't an answer. In that case, I'd vote for the TO_NUMBER option in your SQL statement. The load is put on the DB server, and you're getting data in exactly the format you want inside your model. Also, chances are the server is going to better suited to handle the extra load of changing the string into a number than your desktop is...
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