Hi there I also am about to start my ATPL exams, an exciting tuime in our lives im sure you will agree. In regards to should you purchase second hand books I would recommend not. I was always told that having your own is the best as the second hand books that you purchase are likely to be full of the previous persons notes, bearing in mind your note taking is probably going to be different from the previous person's. I think that for exams that are so important, spend that little more and purchase a shiny new set yourself hope this helps a little.
Regards