Hi Crunchy,

To answer your question on prop-shafts, I usually go to my LHS and look for the correct diameter of shaft I want, since I don't always use the car's stock motor in my conversion.

For the ballast system, you can can have what's called a sealed ballast system, an on/off switch activated by a servos turns a pump which pumps in water to a sealed balast tank through a valve. As teh water enters the tank, the air inside is compressed. To vent the tank and surface, all you have to do is open up the valve to let the air through, the pressure of the compressed air inside the tank automatically blows all the water out.