It seems that you are hoping to implement Payments Standard instead of the Express Checkout option for PayPay to handle credit card processing. This is not necessary as the Express Checkout option will allow your user to pay with credit card or any other PayPal payment method. There are many advantages to using the Express Checkout over Payments Standard, the biggest is that the checkout is a local one and you will know the status of the transaction at the time it is placed.
Using Payments Standard is much harder to work with to get a positive end result and requires a bit of customization to get it working properly with an IPN response. Unless you have no choice in the matter I would suggest that you choose Express Checkout over Payments Standard.
It is very easy to integrate with the Authorize.NET as you can just enable the option from within the wizard to include the Express Checkout. You just need to ensure that you are signed up for the service with PayPal correctly and use the credentials that they provide for you.