Design a cloud based application for smart phone and PC

I am looking to create an application which could handle 50k users from a cloud faced interface. A few points about the application, as follows; The app would have upto 50 categories of different user types, each with a different user journey. The app would control 5-10 unique complex project management workflows, with over 300 unique steps. Each workflow could have 70-150 steps. So the workflows would share the steps. BUt each workflow would have a unique path. The app would need to store and manage many different databases, some of which would be influenced and updated by the users, as supply and demand changed. The app would need powerful API plugins and third party applications for many different tasks ranging from pattern recognition, mining web based unorganized data, google earth, accounting software, pricing software and many others. This particular need could have some manual features for the initial launch of the app. Then as the app evolves then certain input could be automated as the 3rd party apps are connected. The basic principle of the app is that a consumer would come to the app to procure a real life construction project, then manage the process using the app. The app would guide them step by step from start to finish. In order to complete the project the consumer would be using vendors supplied through the app in a virtual marketplace. Similar to the consumer the vendor would also use the app to manage their input on the project and get paid. Due to the diversity of the different types of users there would need to be many different interface dashboards for each user. The app would need to be able to take large payments of over 50k and small payments of as little as a few dollars. The app would be based in the USA initially but preferably be used in other countries later. Initially the app would create micro communities with geographical boundaries for each micro community. SO typically a consumer who wants to build a project in one location would be accessing a list of vendors who specifically service that location, similarly the regulations for that location would need to be considered into each work flow. In summary, I need an experienced architect to design the application and help outsource the development from end to end