RE: How much power is needed ?
Being brushed it will be very tough to find a motor that would be most optimum. For scale boats they still work very well but for FE boats they just can't be matched up as easily.
I would aim for something around 1200+ watts. What motors do you have available?
Usually a boat this size would run on 10s LiPo, at around 100amps or so.
Ryan