Develop E-commerce Platform - Continuous Project

I am looking for developer(s) to complete Phase One of my company’s E-commerce platform. The site will offer monthly subscribers a complete E-commerce solution which will then be hosted on a sub-domain within the platform. The platform will perform similar to Ebay shops but without the auction theme. The platform is to be built from open source code, preferably PHP or .Net – I do not want a license for downloaded software. Please provide costs for website development with designs included and without. Hourly rates are fine but I would prefer a fixed price rate with an agreed deadline with payments being made when agreed milestones have been reached. I understand that much more information will be required, and I have a detailed brief which I can forward to bidders who meet my requirements before asking you to commit to your bid. I already have costs from a development company who employ developers and designers in India; however I appreciate that this company has included their profit margins on top of the developers wage and I am considering contracting out to reliable experienced freelancers. This company anticipates the build time taking between 240-320 hours. Phase One includes the basic requirements to launch the site, and consists of only 20% of what I would have ideally liked included for launch, so there is much more work to follow for the right developer(s) who I hope to build a strong relationship with as the site progresses and evolves.