Doepfer

Hey, I am planning to buy an A-100 system when I graduate very 
soon, and I woould like to find out exactly what modules I will need 
to make a 4 voice polyphonic system. So far, I have decided on these 
modules:
     8 vco's,4 multimode filters,4 vca's,4 envelope generators,4 
frequency modulators.
     What other modules do I need to make it polyphonic? For 
instance, do I need to get several of the midi modules, or just one 
really good midi interfac?
    Thanks