Why would you want to use V3 over V4? V4 has so many new features and also comes with security updates? Why not upgrade?
All the features you require are definitely doable in V4 - not sure about V3 since I no longer use that. I use Amazon S3 for file storage and works great. I can even pull different image sizes for mobile optimization.
Both Heroku and AWS offer load balancing but is a lot easier on Heroku simply by adding another dyno (server instance). Strapi is also starting it’s own hosting services but not sure what that will look like. Strapi also allows for multiple DB connections: see here.
I suggest launching another staging server on Heroku with the customer’s app on it and updating to latest version to see if it works for you.