Hi Evy

One of the existing ways of restricting the courses that users see is to use audience based visibility.  If you create an audience for the external paying clients you can limit the courses to the audience and enrolled users. This means by using dynamic audiences only users who meet the audience rule of external paying users would see the courses in the catalogue .

There are some other ideas around separating audience with navigation, learning at https://help.totaralearning.com/display/TL10/Supporting+multi-tenancy.

As far as the interface for the catalogue goes the Featured Links Block offers some options for adding tiles to audience dashboards

regards