If your reading list specifies a particular version, you should get it. However, don't buy every book on the list - only those which you will consult frequently on the course, and it's handy to have your own copy. Remember most textbooks are only essential for assignments, and provided you raid the library early enough, you don't need your own copy. And of course at this stage you don't know which books will be of more use generally than others. Try to speak to a recent graduate and ask for advice, or post a question here about recommendation of useful books for your subject and phase.