Ecommerce marketplace (multi vendor but single installation)

Hello, I have the following requirements for a multi vendor extension: 1) Can be either Magento or OpenCart based 2) Has to be a single installation (not multi store) 3) Merchants do NOT need to be able to sign on themselves 4) The workflow for customers is as follows: a) select a region b) select a merchant from that region c) select products from that merchant d) pay (not to the merchant) 5) The page where the customer selects the merchant should be similar to this: [login to view URL] a) Customer can filter based on the categories that the merchant sells b) Customer can see description of the merchant along with logo c) Optional: customers can give a star rating to the merchant 6) There should be a different minimum order size based on which merchant gets selected (e.g. $10 for merchant A and $15 for merchant B) 7) This is not a design work - the template will be provided 8) The merchants don't have access to the backend, everything is administered centrally Please let me know which platform you would use and if this would be a development of a new extension or an implementation of an existing extension. Many thanks! and please let me know of any questions! Maurizio