Hi, this is really just my opinion but I think that before you start doing past papers you should go over and fill in any gaps in your knowledge of the content (including case studies) as that will really help you. It would be important since if you start doing past papers before knowing the content fully you might not really know what is going on whereas even if you don't get to the past papers before the exam at least you'll know the content and be able to apply that instead of wasting your time trying to memorise questions you don't know the answer to.
I hope this helps, and all the best for your exam remember that you gave it your all and that's all you can really do!