I\'m in the the process of replacing the Free and Standard paypal button with a Express checkout integration. I believe I\'ve done most of the work but I have 2 outstanding
1.Express Checkout, its actually a API call to the PayPal server.
If you want to edit the Quantity, you need to change the value L_PAYMENTREQUEST_n_QTYm using your own logic and send the request to PayPal(quantity is not editable).Doc
2.Actually PayPal has released a new Express Checkout layout.
There are two URL's here:
Old Layout : https://www.paypal.com/cgi-bin/webscr?cmd=_express-checkout&token=EC-24P528268C112244W 
New Layout:
https://www.paypal.com/checkoutnow/2?token=EC-24P528268C112244W

You can force any layout as you wish.