composing by ear with keyboard/text

Hi-
Any help in telling me how to do this would be welcome. Here is my project. I first want to lay down a track of spoken word. Then I want to write a string quartet underneath this spoken word track. I do not want to put a time signature or a scale notation on the track at first. (If, in the end, the computer figures out what it is- fine) I want to use a cello, and viola, and 2 violins. I want to lay the tracks down one at a time- and be able to listen to them. I will be playing the music, by ear, on an electric piano that is plugged into the computer. Again, the first track with be the text/voice. Does anyone have suggestions? I started doing this with Garageband- but they do not have the string quartet instrumentation- thanks for any help! mpsmps