We are building a large CRM system based on the SalesForce.com cloud. I am trying to put together a test plan for the system but I am unsure how to create system-wide tests. I want to use some behaviour-driven testing techniques for this, but I am not sure how I should apply them to the platform. 

For the custom parts we will build in the system I plan to approach this with either Cucumber of SpecFlow driving Selenium actions on the UI. But for the SalesForce UI Customisations, I am not sure how deep to go in testing. Customisations such as Workflows and Validation Rules can encapsulate a lot of complex logic that I feel should be tested.

Writing Selenium tests for this out-of-box functionality in SalesForce seems overly burdensome for the value. Can you share your experiences on System testing with the SalesForce.com platform and how should we approach this?