Add Paypal to a Sagepay integration

General information for the website: A busy ecommerce site using Sagepay for payments
Kind of development: Customization of existing website
Num. of web pages/modules: 2-3
Description of every page/module: Sagepay has a Paypal module which we want to add. Pages affected would be the shopping cart, 2-3 checkout pages, and possibly amendments to the payment module admin in the backend.

Front end of site is PHP / MySQL / HTML / CSS / JS / Jquery and is mainly using an MVC architecture with some small bits of osCommerce. The backend is largely osCommerce 2.2.
Description of requirements/features: Integration details can be found at http://www.sagepay.co.uk/support/12/36/direct-integration-paypal-payments

This job will mostly cover adding additional lines to the existing Sagepay PHP and XML on the front end of the site on the checkout pages.

We would need a Paypal button on the shopping cart so customers can select pay by credit card or by Paypal on the shopping cart, although it'll still go through to the normal checkout page, just that Paypal will automatically be selected as the payment option if they've selected Paypal.

On the checkout page where the existing credit card payment form is, whatever the customer has selected on the shopping cart as their preferred payment option will be visible, but with the option of changing their selection.

If Paypal is selected, then the standard credit card payment form on the checkout is to be hidden, and customers are instead routed to Paypal when they continue the checkout. Once they've logged in and confirmed their order on Paypal, they'll need to be returned to our site for final confirmation of their order and to complete the checkout.

I'd like an easy way to switch Paypal off if necessary, so whether this is just changing a setting in the code or having an extra line added to the existing Sagepay admin module in our backend.
Extra services needed: 3rd party integration
Extra notes: