I have an existing SharePoint 2013 site and detailed specifications for integrating with an external system using the SharePoint REST API. You will process incoming data from the external system into SharePoint and then update the other system from SharePoint when the SharePoint data changes. Nintex Workflow is available to you. This is a highly specialized skill set and requires experience in this specific area. Please provide your qualifications, rate and availability and I will share the rest of the requirement with you. The budget for this can be significant provided you demonstrate you have the necessary qualifications, experience and aptitude to complete it.
This is only the second project I have placed on [url removed, login to view]; I encourage you to see the feedback left for me on another project recently completed for the same SharePoint site.