Identifying the necessary fields in your core master file profile is a good starting point.  This profile will evolve and change as you add additional features or services to your site.  The one tenet not to be violated is always reference back to this master when creating new services.  One master will enable you to leverage APIs into additional external services with a minimum of effort.

You will have to set up relative translation tables for external resources but as long as you have one master and it is well defined, these connections will be straight forward and easy to create.