Hello Konstantin,
You´ll hear many opinions on this topic. If you know how to build a PC yourself, you´ll be better off concerning the price point. But it´s not an easy task to build a good music computer, which is why some companies specialized in building DAWs. Here are our partners.
I personally wouldn´t go for an iMac, simply because you will be stuck in terms of hardware at some point.
For the Special Edition, you should be fine with 4 GB of RAM, 6 GB are of course better (some overhead for the CPU).
You might even consider using a MacMini in combination with an external harddrive (the internal ones are only 5400 rpm drives).
Many options, really.
Concerning Sequencer: You should use the sequencer you know best (or have some friends that can get you started, or use the one they use at your university)..... On Windows, Sonar is a nice alternative to Cubase, and there are other notation based programs like Sibelius or Notion that might be of interest to you.
What are your colleagues using?
Alright, I guess that´s just the opener for this discussion.....
Best,
Paul
Paul Kopf Product Manager VSL