Consultation and/or building an API around a Delphi .exe App

We have a windows .exe app that we want to create a windows service and run on Azure as an API. It is written in Delphi. I am looking for practical advice from someone that knows the ins and outs of building a robust API. We have a developer that will make modifications to the executable app. Currently the app processes .CSV batch files with 1000's of columns. This will change for the API as it will process 1 record at a time. We want to host the API on Azure and need advice on the best way to do it. Initially, we can spin up a server as needed, but eventually, it will need to be high-availability. There is no need for any coding of the core application. This is more of a developing a wrapper and picking an implementation method. If you have experience using Delphi that is a plus. You must have a clear vision of implementing the API on Azure and need someone with experience doing it. We know that we want a RestFul API and the response will be JSON. Be prepared to discuss the pitfalls of API versioning, authentication and ideas of saving/storing/editing API keys. In addition, we want to be able to measure transactions for customer billing. Due to the nature of this project, it will go in stages. Please discuss your specific experience and how this project is a fit for your skillset. To show you have read this, add your favorite animal in all caps as the first word in your bid. Initial testing will be done on a local apache web server.