Building a sysem like WHCMS is not an easy task. You will obviously require extensive knowledge in a few aspects like PHP, MySQL, and much, much more.
Your coding will need to be integrated with systems such as cPanel and WHM (if your server makes use of it), where you will need to obtain permission and have knowledge of that systems.
You will also need to code a large number of aspects in 'batches' and then ensure that they are integrated in order to have your end-product function properly.
Then you need to contend with issues such as future support. You will be targeted by a large number of hackers who will exploit your system. You will need to identify flaws as they arise, keep developing patches to fix them, etc., etc.
I personally would not be interested in such a system.