Colin, you can barely do any better than the Cardas arm wire, both internal and external. The internal is flexible and the external is almost perfectly shielded. When built together they should give you a great sounding arm. Many companies this this wire, I'm just about to rebuild the external wire for my SME with some.
Pete, capacitance counts for MM carts, more than anything flexibility and low microphony are important and good shielding for the external section.
For me flexibility would be number 1 for smooth tracking.