PHP customization in shopping cart

I need to build an online store. I can install any PHP commerce platform & I will buy a premium theme for my store. What I need from you is: First- Advise me what & why is the best open source eCommerce platform (Magento, Prestashop, OpenCart, est). Not neccesserly to be free, I can pay for that. What I care about is to be open source, so I can customize modify as I need. Second- to do the following customization: I'm getting my products from an existing real inventory (dropshipping), So, I will get a CSV file of the products info (ID, NAME, PRICE, COST, QTY, URL, IMAGE, ... etc). And I need someone to write a simple module helps me to manage this stuff (importing products). The changes will be as follow: 1- Upload CSV products information. This is just to upload the file to my store database in a separate table, up to this step the store products is not updated. And this file is not formatted as the store database. It's exported from my inventory database. 2- Show the imported products, and highlight the updates (New products added, Products out of stock, Products price changes..etc). Comparing with the existing data. 3- Ability to click on "Merge" or "Update", this will move the changes to 4- Products images URLs as in the CSV file refer to [login to view URL], for example one of the products URL is [login to view URL] It should be moved to my domain. 5- Change the Oz to mml. 6- Change USD to SAR (Saudi Arabian Riyals = 3.75 USD). Please let me know how much will cost, and how much time it needs. Note: I set the budget based on my expectations. If you think it should be more, just let me know :) Kind Regards - Khaled