From what I know the requirements are
1) B1900 Type Rating (essential)
2) MINIMUM 500hrs Total Time but it varies apparently...
3) MINIMUM 300hrs Multi Engine
4) Time on Type preferred
Not sure about Visas.
Age for Falcon (info coming from one of their pilots) is min 21 and max 28 for entry as an FO... don't know why...
DHL operate Metros out of Bahrain
Hope this helps