To see most of the free templates in action, go to (sorry, site offline) .
Support for Zen Cart is provided principally through this forum, by volunteers (including the developers). You can find some people and companies that will provide paid support by looking, but nobody is permitted to advertise here. Read the forum to see the number of longtime users who help others for free; I think you can consider them happy with the platform. Given the total number and experience range of users, there are bound to be a lot of people posting with problems or questions. What is relevant is whether their problems are solved.
Most templates can be installed by anyone who knows how to use a computer and is able to follow instructions. Modifying a template may require some degree of familiarity with HTML and CSS, and sometimes PHP. This depends on what kind of modifications you want to make. The forum is here for assistance with this.