Personally, I don't put much faith in DVD learning, esp. if it is a system in which you don't have any prior experience.
I think your time and effort would be much better spent working on your kenpo. If you want to study something else, find a teacher. If DVD is your only option, I would suggest you don't do it. Save your money and focus your efforts and time on what you already have, and make that as strong as it can be.