I recently did this on my Gulfstream - it's a great addition.
The most basic system will require an autopilot computer (the black box), a compass that can talk to the black box, a hydraulic pump that is plumbed into your steering, and some sort of interface (to engage/disengage and set parameters). Some chartplotters are capable of providing the interface but I am not familiar with the hummingbird products.
The pump needs to be sized to match your system. If your twins are steered by a single ram with a tie-bar to the second engine, you should be fine with about a 9 cu inch capacity. Twin rams double that. On my installation I put the pump in the wire chase under the helm with ~3' hoses that are tee'd at the helm pump.
Basic systems are available from SiTex, Simrad, Furuno, Raymarine and others. The SiTex sp110 is probably the best bang for the buck for a standalone system. I went with a Simrad unit because it integrates with my chartplotter.
Good luck!
