OpenCart Product Page and Checkout Customization; Features & Interface

I have OpenCart v1.5.4.1 installed already with Shoppica v1.4.5 theme but I need to customize the product page to match my type of business and products. I have a print shop and require a product page that is dynamic on the front end. It will have to change options/pricing on-the-fly (without reload) based on customer's choices and input. For example, if they are ordering a banner, they will need to specify the height and width, materials, etc., which will impact the pricing shown. Likewise, the back end (admin) needs to be modified accordingly. Site performance and SEO is also important so work must have positive impact to both. This should be a fairly simple project for someone who knows what they are doing. Example of similar features and user interface: [login to view URL] Please show me examples of past work and scope of involvement.