It’s a good thing that the Destinos course includes the Spaniard as well as the Mexican pronunciation of Spanish, that way the student will be able to recognize the differences in dialect right from the beginning.
As to caro, Luke, it is very rare here in Mexico. We usually use “estimado(a)” or “querido(a)”, but this last one you use it when you know the person you are talking with very well (family member, close friend, girlfriend).
Saludos.