Fast Checkout

I've searched through the forums for existing posts on this, but haven't come across any solid information.

How can I add a "Checkout with PayPal" button so that customers can bypass entering their shipping and billing information, because all of that is (should be) held on their PayPal account - and that they can just pay with their PayPal balance/the debit card(s) attached to their PayPal account?

That's called paypal express and you can buy a module for it from Q. PAYPAL EXPRESS INTEGRATION