Exigo Web services API- Can use with Wordpress or .Net or java which are stack capable of posting HTTP under SSL; So an easy task for me to manage.
As per my knowledge, Exigo Web Services provides Customer Management Protocols; all transaction function module; recurring order details, Items details, Payout module, reporting module, Communication, party plans, Points or Subscriptions, Calendar, Customer Leads module, Extended DB module and many other. Please let me know whether you need all the modules connected with your website and fetch data? or need only customer data?
Customer ID, Login details, All details of addresses etc be fetched through the web Services API. Easily manageable by me.
From my previous experience: may be difficulties to get the credentials of the Web Service API; The solution is to use XML/XMLParse to fake the authorization header entity and adding it to the web service call using the addSOAPRequestHeader() method;
Sendgrid API endpoint allows user to send email. It also provide libraries to help users for easily integrate with the v3 Web API in various languages like PHP, Java, C#. Easily manageable;
Regarding existing plugin customization- Need to know the details about the APIs and what changes are required over there? Please ping me to start a discussion. After a short discussion, I can start to work with this project. Available now;