I developed the PayPal Buttons module for a client who wanted a similar workflow.

The module allows you to create a PayPal Buy Now button on a product page (that can replace the 'Add to Cart' button), which enables the following workflow:

1. User clicks 'PayPal Buy Now' and is redirected to PayPal for payment.
2. User pays on PayPal and is redirected back to the store site.
3. PayPal sends an IPN (instant payment notification) to the store site, which confirms payment, and optionally can include the billing/delivery address.

Currently, this does not work for anonymous users. I would accept a commission to add this feature to the module (which could operate by registering the user by their PayPal e-mail address).