I am planning to use WP& a premium theme. I want to customize "pages" to show data from a DB and also their design regarding to show this data. I am a newbie that's why I am not sure if only creating a custom page template is enough for doing this or a plugin is a better option.
You won't need a plugin for this. You can either a) modify their template (for example page.php) or create your own (page-custom.php). This will help you: http://codex.wordpress.org/Template_Hierarchy.