Opencart Multi-vendor system addition of features

I need some changes to be made to the opencart software to allow my site to have multiple vendors that share the same store. So, it would sort of be like Amazon where merchants each offer a book for sale and you choose whichever want you want to buy from for any particular book. However, unlike Amazon, I want it set up so that the vendor who is selling the item for the lowest price is the only one that shows. Then, when that vendors items are out of stock, it will go to the next lowest vendor, but first it will send him an email letting him to confirm that he is ready to supply those items at the price and quantity he indicated. Until he responds to that email, it will show the item as out of stock. Here's a list of the feature's I'll need: 1) All vendors share the same store. 2) Vendors have to be approved before they can sell their products either through admin approval or through a verification code that they enter upon registration. So, for instance if the verification code is 2lkj2342oh they have to enter that when they're registering and if the verification code exists in the database, then they can instantly start selling their items, if not they have to wait for approval from the admin. 3) Lowest priced item always wins. So, if two vendors are selling the same kind of shoe and one is selling it for $23 and the other is selling it for $34, only the one selling for $23 will be shown to customers; the other will remain hidden until the one that's selling for $23 runs out of stock; upon which the item will show up as being out of stock and an email will be sent to the vendor with the next lowest price informing them that they have to indicate that they are capable of selling the item at that price and how much of it they have. 4) Same checkout if a customer buys items from multiple vendors. It would be a flat rate shipping but each vendors shipping would be totaled up for the customer upon checkout. Here's an example of what I mean: Customer buys a shoe from Vendor A for $23 and a hat from Vendor B for $12. Vendor A and Vendor B have flat rate shipping of $3. So, each individual charge and the total amount is calculated for the customer showing a final price of 23+12+3+3 = $41. 5) There should be a way for vendors to enter the shipping information to let customers know when their product will be arriving.