One problem you will face is getting your data to be consistently correct. I'm sure no 2 clients will have the same types or data. I would think without creating your test data you could end up dealing with a lot of brittle tests( or more of tests work on one clients system, but not anothers due to missing data or different expectations)
I dont think you understand what BDD is. BDD is a style of software development. A BDD Collaboration tool like Cucumber or jBehave. help you write out examples of what you are building in plain english. They still depend on a Testing framework to help them make those examples executable. Most BDD tools require you to use assertion lib's from a testing framework. TestNG is a unit testing framework.