You should also get the book 'Model Submarine Technology' by Norbert Bruggen (also by Traplet), that goes much further into the nitty gritty technical stuff.
It doesn't really matter where you site the valve, just so long as there is one there. It can be between the pump and the outlet, or the pump and the bag
The type of valve you need- there are three possible approaches.
1. A servo controlled pinch valve. You run silicon tubing to your pump, as this is very squishy it can easily be pinched up. Using one of the servo arms as a cam, the hose can be pinched off using either a blunt plate (think guillotine, but not sharp) or a ball bearing. If you use the latter method, you'll probably need a little more pressure, so it's usually advised that a standard sized servo is used and that it's ballraced. I use the guillotine type pinch valve with a little 9 gram servo. Works perfect. You need to make sure the hose is unpinched when storing the boat, else the hose can get damaged.
2. A ball valve. Festo do miniature ball valves which are designed for pneumatics, but work equally well with water, as the materials are non corrosive. You have to modify your servo to operate over an extended 180 degree range to use these. The valves cost about £8.
3. Solenoid actuated valve. OTW use a solenoid valve in their modules, but it's quite a bulky item. If you have the room, no big deal. Some have used Clippard valves, which are quite small. Unfortunately they're designed for gas not water, and they corrode if you don't strip them down after a sailing session and dry them out thoroughly.