I see that you need rather complex solution. The one to import through API supposedly will be custom built according to your needs and be highly expensive.
I can offer you ready-made solution and you yourself decide whether it suits all your needs.
There is the application called Store Manager for Magento along with multiple addons which extend its functionality.
Using Store Manager for Magneto and Automated Product Import addon you will be able to set up automated scheduled updates of your store directly from suppliers. Using definite formulas it is possible to import product with the lowest price in case multiple wholesalers sell the same products. For example if you have file from supplier A, B and C you need to run first import from file A as is. While importing from Supplier B use expression formula that checks product price and skips values in case the price in the file from supplier B is higher than it is in the database. Same procedure should be made with supplier C, thus replacing products from A and B if the price is lower in the file and so on and so forth.
Specific margins and price mark-ups are also possible to add on-the-fly during import using definite expression. Lets say, you need to increase the file provided by your supplier by 15% right in the process of import, so you need to use this formula -
Basically, any data modifications are possible using some rules and formulas.
As for integration with Icecat, it is also possible, as there is Store Manager addon called IceCat Integration that allow to update your products with images, descriptions and other product details from IceCat.
Anyway, there is 2-weeks free and fully-functional version of Store Manager for Magento available for download (addons built-in there in demo mode)-
If you need assistance with the software, let me know and I will guide you through.