If you just want that content on your homepage, you can insert it into define_main_page.php in Tools > Define Pages Editor.

Are you saying you want the Add function to go straight to checkout without the opportunity to do other shopping? The default Add function will go to the shopping cart content page, with a prominent Checkout button. This could be customized with good PHP knowledge. Of course, the content and styling of the cart page can be customized however you want.

I think it will take more work to avoid the standard Zen Cart page output and use totally custom content on a page, than to strip down the ZC layout and use its built-in dynamic abilities. You can turn off the sidebars and some other elements in admin, and header elements can be reworked easily in /includes/templates/your_template/common/tpl_header.php.

If your ads for the homepage are the basic kind of "product" content you will have, you can set up a category with those products to display on the homepage, set to display only the product image and add button, and with absolute positioning spread the add button over the image. This will let you handle new products with Zen Cart's built-in admin tools.