Hi,
I see a problem of logic in your request.
What do you do when you have both a normal product and that special product in an order ? Do you send the special email or the normal one, or both ?
If you just want to add text to the normal email if (only) the special product is there, then it's easy, with only 2/3 lines of code in the code of the email that you can edit via the menu System>Emails:
<?php foreach($data->cart->products as $product){
if($product->order_product_code=='My product code') echo 'My custom message for the special product';
} ?>
Otherwise, sending a separate email would be quite complex and it would require custom development. In such case, you can contact our partners for a quote on the job:
www.hikashop.com/home/our-partners.html