Customize Gallery2 Checkout Cart

Need Gallery2 photo checkout module customized to do real time photo pricing. Rather than the standard size / price and Add To Cart, want user to be able to specify the size of the image to be printed and the site will calculate the cost based on size, cost, handling and markup. Also want to be able to offer additional services like photoshop and stitching. The fields are entered and calculation made. For custom. The print pricining would only be shown to the owner of the image and the option of the fixed sizes / pricing to remain. New fields would include size1, size2, paper, border, comments and then additional services. The requested fields to appear below the image. Price would be updated and add to cart button. Then, in view cart the correct information will be presented and sent to paypal. Want customization documented for future expansion and not interfere with other modules. Attached is a screen shot of how I would like the page to appear on the order page with the requested information. An "Add To Cart" button would appear when all the required information is selected. Javascript checking of fields is desired.