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.

I needed this done last week so am in a rush to get started early next week. I'll be checking responses to this project over the next few days and choosing someone to work with by Monday.