I have a single 5745 hitec on steering, but since I have now added cable brakes it isn't quite as fast to respond as before (the cables provide extra resistance on the steering).
So if you don't intend on front cable brakes, one hitec would do, but if you want them, you may like to consider something with more oomph, or two hitecs on the steering.
I'm going for one 645mg on throttle/rear and one 645mg on front brakes, but I have not yet wired up the 2nd servo for the front cable brakes as I want to knock up a harness to bypass the receiver for servo power (bits are here waiting, I just need a spare evening. I know a Y-lead or two would do it, but I want it to be a bit neater with a voltage regulator as well).