Consultation needed for creating an IPhone/Android App and Website

I need consultation (only consultation at this point) on how to build a fast and scalable website and App (IOS and Android). The Consultant/Freelancer must have extensive experience in building e-commerce related websites and apps. Must also have extensive experience with using public cloud for this purpose. What is going on: a large and growing amount fashion-related images are stored in cloud every day. Android and IOS apps and the website gets the images from the cloud and the indexes of these images (images are indexed) from apache Cassandra. The apps and the web also get user information from a relational database. All of these components should be in the cloud. The Website and the app are accessed by large and growing number of people daily. Here are some of the points I need your input/consultation with: 1. The Website and the Apps have to be extremely fast. You will suggest ways and benchmarks to achieve this. Suggest 2 options and argue for pros and cons. You should also suggest which one to build first (apps or website) or it doesn't matter? 2. Cost is also an important factor. You will suggest ways to minimise the development cost and the cost of running the apps and the website (the cloud costs). You will also have to suggest public cloud/s and argue why this suits better. 3. You should make your recommendation considering that both the website and the apps will be used world-wide (in many countries). Suggest ways, how to achieve requirements 1 and 2 considering 3. 4. You should recommend architecture for apps and the website so that they will be highly scalable and resilient. 5. You will draw a detailed architecture diagram for the apps and for the website . You should also write a (2-4 page) document for achieving points 1-4