As a longtime CoC player and Keeper, all you absolutely have to have is the Corebook. The newer editions 5.6E and up have a fairly good information for the modern era and all the Mythos information you can handle. All the best supplements are long out of print, but since you stated you wanted to do modern era, its not like you need to know about music, cars, weapons, or laws of the 20s. However, the two volumes of the Keeper's Companion are extremely useful and happen to be in print. Other than that, you do not need to run out and purchase a mound of books to have a successful campaign.