Hi Will,
I would hide the external system behind a Model. I view Controllers as those things that manage the interactions between the ‘things’ in an application. An external system, as it appears, is another ‘thing’ in your application.
This gives you the ability to use this Model in other controllers, or other applications should the need arise.
However, I’m sure you’ll get plenty of options from this list on which to choose a solution that fits your needs.
Cheers, Mel Riffe