Hi,
How are you? I have checked the website, actually it's developed in classic ASP, not in .NET.
Regarding the responsive development, I don't think it's complicated, what we have to do is :
a) First need to create Home page and the category wise landing pages (eg. Filme, Mieten, Musik etc.).
b) 2nd step would be creating "Products display" pages for each section. This page will be again unique for different categories. (Along with the submenus)
c) 3rd step will be creating the "Production detail" page, this i think we can keep the same for all the categories and products.
d) Next step will be creating the order pages and subquent other pages. (common for all)
I will send you more detail for each steps mentioned above once you agree to the above proposal.
Look forward to hear from you.
Regards