Well, my impression is that you would do well to have a notebook computer. However, you shouldn’t feel compelled to get one. You would probably be taking notes in lectures, and possibly in seminars later on. This can be done entirely by hand, and some might say that that’s preferable; however, it wouldn’t be out of place to be using a computer for that sort of work.
My own experience is a bit different. I’m reading for a humanities degree for which I have to write weekly essays and translations. Our essays are all but expected to be typewritten, although with my current tutor he never takes my work in to read it, merely listening to me read it out and commenting during the tutorial discussion. In lectures I never have any need for a computer, since I find it easier to take that sort of note by hand (although I’ve mostly given up on going to lectures for the moment, but that’s another story). In many ways I prefer working in my room than in a library, so most of my work gets done on my desktop machine. I have got a laptop as well, though. If you will be doing lots of work in libraries, either because you can’t check books out, or your room is too noisy, or you get too distracted in your room, then a notebook is a good way to go.
It’s very reasonable to worry about making the wrong impression when you show up, however I don’t think that bringing an iMac would necessarily do that. There are various things you should check, though, such as the size of the desks available in your accommodation, and the level of security available. You will almost certainly want to get possessions insurance, such as that provided for students by Endsleigh.
Hope that helps, and hasn’t just added to the confusion. In some ways, it might be best to try out having the iMac for a term, and see how you go, and then if you feel like you’d do better having a notebook as well or instead, then do that during your first Christmas vacation.