Greg, I think the best way to go about this is to look around and get a mag with a free boat plan in it, and this will give you an idea of the layout of the various bits involved.
It depends how much you want to spend on the radio but a basic 2 channel 27Mhz set would suffice.
You need an Electronic Speed Controller ((ESC) for motor control back/forwards, a 6v 4.5 Ahr battery..or something lighter like a 7.2v 3700 MaHr NiMh battery and I should imagine an MFA 385 motor, from Maplins for instance, coupled with a coupling to a 35mm prop would provide adequate go. It wont be very quick, but the current drain would be minimal and so the battery stands a chance of lasting the day on one charge.
How long a propshaft you need (4mm shaft diameter) also depends on where the motor goes, and the layout of the hoppers. A medium sized brass rudder and tiller arm is required.
The hoppers could be released off catches opened by a suitable linkage off the rudder servo when pushed either side on the trim switch
A 4.5Ahr gel battery weighs about 720/750 grams, a Nimh one half this. The various bits need to be positioned on the boat with FULL hoppers to get the balance right. Thought also has to be given to balance once ONE hopper has been dumped. Ashley