Get page content by ID (from a plugin)

The values assigned to $user_set_value should be stored somehow in some form, usually an option. You will know how the values and where the values are stored. It is easy then from there

$user_set_value = get_some_saved_option_value();
$args = array(
    'page_id' => $user_set_value
);

$q = new WP_Query( $args );

Just change get_some_saved_option_value() with the actual method to get the value.