I answered it in the support portal. Here it is again, just in case you didn't get it:
You will have to modify
ez-delivery.php to verify the transaction and hook up your license key generating module.
ez-office is the IPN listener. Here, you will see where the customer email is sent. You will hook up your key generation before the email and send the email with it.
ez-delivery is the customer return page after payment. Locate the place where the page source is generated and include your license key there as well.
You may have to modify
ez-shop.php, which is the shop front end. The table generated by it will need to contain a link to your free version of the product.
This I guess is as far as I can guide you without actually looking at the code and giving you the actual variable names and line numbers (which will be like me writing the code, wouldn't it? :-)
And no, you won't need the pro version to do these things I just outlined. The pro version gives you a bunch of other features though, which you may need later on.