Redirect to an external site when accessing the page of a node meeting a set of condition
I would use hook_node_view.

Remember to check the view mode along with the conditions, so you don't redirect on pages where only the teaser is shown.

It should look something like this:

module_node_view($node, $view_mode, $langcode) {
  if ($view_mode == 'full' &amp;&amp; "Conditions") {
    drupal_goto('http://example.com');
  }
}