Converting E-Commerce Website to CMS and PHP-Cartsystem

We are reseller of products for creative people. we sell painture, paper, canvas an so on. There are about 12000 products at our site. We are using a 11 year old shopsystem that is based on html and javascript. So we need a complete redesign of our website. We thought it would be good to get typo3 and tt_products as basis but we are open for suggestions. there are two important things. First, we are selling inside germany, so we have to take advantage of the german law espaceally for a law that calls "Grundpreisauszeichnung". We must calculate a second price called baseprice. In example 200ml water costs 1 euro, the baseprice per 100ml is 0,50 Euro. The programm for the calculation with all baseunits and regulations by the law is allready written in javascript. Second, we need an interface to transfer data from and to the website to our faktura-system. This is easy when we use mysql as database for the cartsystem. We want a new design based on css with 2 menu-levels at the top and a third at the left or right side and it must be possilble to set special blocks for expanding the functionality of the site. i.e. special offer, best product of the moment, poll-systems or anything else. the look of the site should be leaned against the current design but it needs an freshup.