You can do any type of integration even with our trial version since we provide all of the api's and capability that you see with the demo site that comes in our eval package.
You can learn more in general about the overall architecture as well to give you a high level picture and of course there is a number of documents and guides we have to help you through the ecf.
As far as source code - we have 2 types of licenses. Front End Source code that comes with a developer seat as well as full source code which provides you the source for the front end, the backend, and the framework API itself.
No matter what, you will need a developer seat license for each developer who is working on your project as well as a deployment license when you go to production.
To answer your question more specifically though, it depends on what you want to do. The ECF is designed to allow you alot of choices in your integration and your own business needs.
Depending on the level of customization or integration you require, you may need the full source code or you may not.
In addition, we have released a new companion product for ECF called ECF I/S - the I/S stands for integration services and it is another web Services framework specifically designed to enable 3rd party integration and back office integration for existing/legacy systems, or OTS ERP systems like MS Dynamics, SAGE, SAP etc...
This new product is only available in a full source code license.
Hope this helps and let us know if you have any other questions.
New to ECF Fast Path Guides...
Demo Site/Starter Kit to Use as Base looks like this in our distribution