Design integration and customisation of existing oscommerce

We have an existing heavily modified version of oscommerce. We also have a new design. We need to integrate the two. We can provide a copy of the existing site as well as and an html version of the home page look of the new site. we can also create all the images/buttons etc required. We also require some customisation around categories/subcategories. when a category is clicked we want a page which splits the subcategories down whic brief information about them, featured products in that category, top 5 bestsellers in that category and then a more button to view all in that category. We can provide an html page showing how we want this to look and all images/graphics. If successful we have an ongoing requirement for maintenance and customisation to our 2 oscommerce stores, plus we hope to do the same kind of upgrade to our second store in 2010.