The exam is a little bit difficult without participating to an official course. But it is not impossible. I have received emails from people that have passed the Spring 5 certification exam by working with my previous book(which was writen for Spring 4) and filling up the gaps using the official Spring Documentation reference.
I can confidently assume at this point the 2nd edition of the book, written for Spring 5 will be even more helpful.
Nonetheless Spring changes fast these days, so as long as you keep your eyes on the official Spring blog and the official Spring certification page and go throught the official study guide throughly, you should be fine.
As for buying the 2nd version, it really depends on you. (Of course, I would preffer for people to buy this book, there is a whole team of people that put a lot of effort into making it and we deserve to be rewarded for that.)
But in the end these are the questions you have to answer before making a decision: do you feel confident in your knowledge? Are good at organizing information and finding what you need to fill in the gaps? Do you have the time to do it yourself? (because the 2nd edition might reduce the effort) If the answer is "yes" to each one, you are awesome, you've got this! and you probably do not need to buy the new edition of the book.