The software takes up almost no space at all. What you need space for is the audio files that you record and mix - they take up a humongous amount of disk space. One thing that many people do is get a machine with two drives, and dedicate the second drive only to record and store audio files. It's a really good idea. Get drives that are as big as you can.
As for the rest of the machine, get one that has as fast a processor and which has as much RAM as you can afford.