magill : I'm assuming your steering an outboard & not an I/O. If that is the case there are several good options for the cable steer you have with out going the extra expense of hydraulic. As mentioned above Si-tex makes a good basic system utilizing an octopus R wheel drive, your rack & pinion cable will have to be replaced with a rotary style cable to work with the new helm. The Lowrance/Simrad system utilizes the same octopus helm as the Si-tex. If you do decide to go with hydraulic steering stay with the Seastar line over the Baystar or other after market brands for compatability with add ons later.
And Chas : It depends on the system . The octopus is capable of driving some i/o's without the engine running.