As far as I can tell, the minimum here is:

Mexico (best way to get Canada)
Brazil (gets SA)
Panama (gets the remaining Central American countries)
Haiti (least characters for DR/Haiti)

Spain (gets Ireland + Portugal)
Austria (gets Denmark + Italian city-states)
Russia (extremely useful)
Bulgaria (gets the Balkans and Turkey's borders)

Iraq (gets Middle East)
China (gets Bangladesh + rest of mainland SE Asia)
Malaysia (gets the islands in SE Asia)

Namibia (gets all needed in Southern Africa)
CAR (gets a lot of Central Africa)
Mali (gets basically all of North/West Africa)
Sudan (gets Horn of Africa)