You can create a template page and get content from that page. You can see an example of this in our User Registration Solution or the user contact form solution.
We use code in the email body similar to:
<?php
ob_start();
require_once(dirname(__FILE__) . ("/Email_Templates/contactus.php"));
$contents = ob_get_clean();
?>
Then you can refer to the same template for mulitple email bodies.