Corporate API for external and internal services

We have many systems that are master data sources, most can be accessed by SQL but a couple are via web services.

We're a Microsoft house and would like to create an API that has some logic and to aggregate requests together and also have a client portal talk to.

Ideally we'd like operation admin staff to be able to change the logic without having to change any code. We don't want to use any cloud based services and would like it on premise, we want latency low as some calls are going to be made by a machine line.

Could anyone recommend an approach to solve this issue?

Many thanks for any help

Si