custom plugin written for opencart

i have a customer group that i will give a special discounted rate to. that customer group will come into the open cart store and take all the items they need and place them into a wish list, which can be shared out with their buyers. their buyers will then come to the opencart store, enter a code to retrieve that wish list, and purchase it. if their buyers have a discounted rate as well, then the discounts need to accumulate, and allow the buyer to purchase that wishlist at the final rate (retail price - group rate discount - buyer discount = final price). need to be able to control discounts per user group. need this written as a plugin to open cart, so it may be portable and not restrict future upgrades.