Product import to Magento using several Excel tables of data

I have several Excel sheets of data which were exported from an existing (non-Magento) e-commerce website. The data represents product and category information for thousands of products from several 'stores' / domains. Each of these will correspond to a Magento 'store' on the same installation / same backend database (this is a standard Magento configuration so anyone who doesn't know what this is please don't read further)

The products are items of clothing with SIZE and COLOUR configurations. Data includes URLs to images and this string needs to be manipulated so that we can use it to point to the same images in the new Magento store. Each product can have several images.

The data in Excel needs to be imported to a clean database in order to query it and get it into the correct format needed to import into Magento. The Magento products will be 'configurable' products with size and colour options. Again if you don't know what this is, stop reading. I need someone who already knows how Magento works.

-----------------------------
This is the second time I'm posting this. If you do not FULLY understand each and every word of this project, please do NOT bid. This is not an easy task and requires someone to have advanced SQL query knowledge and advanced Magento knowledge. This job cannot be done manually. You have to use SQL queries to manipulate the data. If you have questions I'm very happy to clarify.