You would first need to run the checkout wizard to create your PayPal checkout pages. You can use Payments Standard with IPN, or you can make use of Express Checkout. The Express Checkout is a local checkout, the user clicks on the pay with PayPal button then goes to PayPal to login. Then they go back to your site to a confirm page. When they confirm the details the transaction occurs directly with PayPal and the user will be sent to the success or failure page. This is very different that payments standard where you have to use an IPN page to get back the details of the transaction.
If your client has a PayPal business account with the API credentials available to them then I would suggest making use of Express Checkout instead of the Payments Standard.
In either case you will need to generate the first set of checkout pages, then once they are complete and tested to be working you can then craft the other checkout page. You can use the checkout wizard to create these pages, then after they are created you can combine the checkout form on the new checkout page with your paypal checkout page. This part would need to be done manually to make it work this way. Other than this the extra set of checkout pages will be created by the checkout wizard and you should not need to modify them.
