I changed to hydraulic steering on my 9 ton with a Vetus hydraulic system. Space is at a premium at the back of a Hillyard and you need to fit quite a powerful ram which needs to be garaged. We chose the MTC125 ram with the MTP42 (or maybe 53 but can't remember) pump giving about 3.5 to 4 turns lock to lock. Make sure the pump is the one without the non return valve assembly. Vetus will do the calculations for you if you give them the rudder dimensions etc.
Aside from the first half hour of panic because the steering was so light and different from the old cable system I am delighted with it now. If you are really interested and want to talk it through pm me.