Hi, This sounds basically workable, including a LOT of work from you...
You won't find any shield that will do all you need; think about getting separate Stepper and power control parts, and possibly opto-isolating them from the Arduino.
You;ll have a lot of connections and cabling. Consider a sensor shield like these:
http://arduino-info.wikispaces.com/SensorShield
These are passive, but bring all connections out to easy-to-connect 3-pin connectors. There is also a Mega version (scroll down)..
What are the details of the steppers: power levels etc...??