The system you are trying to create is not going to be a simple project, it will require a fair amount of knowledge of how a database works, how the paypal transactions work and how to update the database after the transaction has been completed.
if you don't have a good understanding of the eCommerce workflow, and PHP / MySQL, this will be a very tough project.
I'm not familir with x.com, so cant really comment on that, but i think what paypal is trying to say is that payments pro alone wont be able to handle tracking the user accounts. the unpaid balance, and deducting the amount paid when a transaction is made.
This is the part you will need to develop using a secure login area to log the user in so they can see the account balance, and make a payment.
Paypal Pro can be used for processing the payment, but you will need to develop the rest of the system that tracks the users account.
