Things to consider while developing beta version of your web startup?

I am working on developing beta version of a web/mobile startup that will act like a middleman between Retail stores(of a specific niche segment) and consumers. It will let the consumers search for stores based on products they offer, location, price, etc. and purchase/order them online. Store owners can maintain their catalog, pricing and other details and can view/process their orders online. They will also be provided an autogenerated site/profile(storename.sitename.com) with the capability to use their own domain name if they want. Later, after version 1.0, a mobile application and an API for integration with their existing Retail Software will also be provided. Now my question is how you go about developing version 1.0 of such application apart from core functionality. I mean things like security, testing, performance, hosting requirements, scalability issues etc? Any other advice/guidelines from technology perspective? I am using LAMP(PHP) platform for this. I know python is better but I don't know the language and though I can learn it quickly I don't think it will be a good idea to do something serious as learn&do project.