If you were planning to use the C64 for music production you definately should give the AHX-Tracker a shot, that generates "chip music" sound rather similar to the C64 SID chip.
There are a few clockport soundcards for the A1200 you should be able to get second hand, such as the Delfina and Melody, at least the latter with excellent sound quality (separated digital/analogue PCB for ~108 dB S/N ratio). This makes a real difference to the 8-bit ~28khz samplers you can get for the parallel port.
The Amiga can read and write 720KB PC DD formatted floppy disks starting with Kick 2.0, if I remember correctly (Mount PC0) -- this should get things transferred. Other means would probably need expanding the Amiga a bit (e.g. building the A1200 into a tower and getting a network card).
