Build an Online Store

I have Multimerch 8 system in OpenCart 2.3.0.2 - which allow vendors to sell products - I need to customize it: ♦ Shipping System: When vendor makes his account, system asks him to fill his own shipping methods and prices ​ so when customer order 4 deferent products from the same vendor, he won't pay 4× shipping price, he pays only 1×shipping price and 3× additional products shipping price. (e.g: if we said that shipping costs 500Y.R and additional item costs 50Y.R so when customer order 4 different product it tells him shipping from this seller will cost 650Y.R rather than 2000Y.R which is now) ♦ Category Attributes: when vendor selects the product category it shows him the attributes related with the category he selected (e.g: when he chooses category "Laptops" it shows (Brand,CPU Type,CPU Speed,RAM Size,...etc) , when he chooses "Mobiles" it shows (Brand, SIM Slots, OS, ... etc). This extension will help you to understand what I mean: [login to view URL] Also I want the MultiMerch 7 attributes system to be back so vendor chooses from attribute's inputs predefined by the admin ♦ Vendor's Auctions: I want vendor to be able add an auction for a product he uploads (with starting bid, time limit... etc) I found this extension that may help [login to view URL] ♦ Vendor's Coupons: Like OpenCart coupons system, vendors will be able to give their costumers coupons to be used for discount, free shipping... etc. Vendors will be able to create coupons that may be used once or for x times and between date y to date z Freelancer will need to deliver I want that to be done in vqmod so I can update system in future without future modification.