I'd suggest checking the
International Postal Union web site. They have enough discussion of the issues to give you a good idea of how to cope with the addresses you might encounter.
Just FYI, pick a sub-set that will cover most of the addresses that you'll need at first... Trying to create a "universal address" database is a huge task, that rarely pays off very well.
Very few businesses truly need to be able to address mail to everyone on the planet, and they usually already have the address expertise needed.
-PatP