Actually just to add an interesting bit of information to this question, I am preparing for the GMAT and happened to have an old 2007 edition of Kaplan's prep book as well as a 2012 edition of Kaplan. After going through all the practice questions I found that the questions between both editions were exactly the same, except for maybe a handful of questions. The rest of the questions were exactly the same to even the order in which they appeared in the practice sections. In terms of practice questions that 50 each in critical reasoning, sentence correction, reading comprehension, problem solving and data sufficiency. So only a handful different when comparing 250 sets of practice questions. Granted I haven't done the practice exam at the back of both books, but I'm guessing I'm going to see the same pattern there as well. Also there was hardly anything different between the advice and strategies of both editions.
Makes you really think about the benefits of getting a new edition book. But yes, the only the latest editions will have anything about the new IR section. So if you use an old edition for everything else, even if it's (at least by my sole observation) 5 years old, you should be fine. So I would say no harm in going with an old edition, and a dedicated IR book.