First of all we need to define business/user values. I would research following:
Getting answers on these questions could build a common picture about predisposition in our case to create an API on our side. For example, if our clients have an API (for their ERP) with a doc and we can integrate them manually it could be kind of MVP in this situation. Manual integration will allow us to validate some hypotheses on work with largest clients and datas we need to work with to provide the best experience.
Primarily, we need to define the most important data we need to implement in the API. We can obtain it via interview with our clients and research ready-made solutions in the market.
After we get knowledge of what we need to do, there will be a few main milestones:
In doing so, the more down level decomposition we prepare on the developing step the better and more manageable the process will be.