Complete e-Commerce site with SEO

I have a website that is half complete. The design has been done, and some of it implemented in PHP and HTML. I need the e-commerce aspect of the site to be completed. This includes the shopping cart, setting up the payment gateway etc. I am also open to the idea of redoing the entire site (keeping the current design in place) using a tool like OSCommerce or any other free e-commerce system that you can use, as long as it is within my budget. Please use the PMB if you're genuinely interested. Also type trimandtassels in your bid, so that I know you've read this. My budget is < $200.