Hi there Ferrets,
I have the same boat, given to me by a friend in a disreputable state years ago now. After rebuilding and repainting the model, I originally had it running with one ESC linked to both motors, but the turning circle was about half a football pitch in that configuration! I then bought a second ESC and mixer from ACTion, at that time run by Craig Talbot, now run by FLJ on these pages. Net result was that I won my class of Scale Steering (South West England Area) from five different venues around the area. By the last round I only needed to turn up to win the series, but in fact won that round too! My boat runs on 6v SLA, 4AH, and needs no further ballast, on 2 x 380 motors. Yes, you will need a steering servo, but with a mixer, a 2 channel set will be enough, but more channels if you wish to add extras, like lights, sound etc.
HTH Danny