Buy it as soon as possible and start studying for it as soon as you get it, honestly. If you study for like 2-3 hours a day, until the pcat, you'll definitely ace it. Start taking pre-tests like towards may-july to gauge where you are. I bought mine a couple of weeks after the Nov pcat and it didn't come with the update from the nov pcat. It was sent separately at a later time. Honestly, most of the questions are from like years back (2011-2013) so buying a new pack isn't really necessary and most people (including me) have the packs that we don't even need anymore.
I personally studied for 6 hours a day for 2 weeks and got in the 60s after taking it the first time without studying and receiving a score in the 20s.