There is a google checkout module in the downloads section, I know there are several people working on this and I believe it is mostly sorted out. This module uses the cart and sends all items at once to google for completion, allowing more than one product at a time to be purchased. Check this thread for where they are on this,
http://www.zen-cart.com/forum/showth...oogle+checkout
As far as writing each page, adding buttons and updating them manually you would be better off using the cart to produce each product page dynamically. One set of files to keep updated and the change reflects throughout the site.
The registration process will not be an issue since google checkout is an express checkout and will process the transaction on the google site using the customers googlecheckout account, there is no reason they would have to register, but if you leave it in place they could have the option of registering and the added preks that go along with it, progress tracking, order history, newsletters etc.
On another note I believe the buy it now buttons you are using from google are setup on a per product basis, therefore the html code would likely be different for every product, one more maintenance nightmare.