I'm assuming that the probes are level sensors that stop the pump and close a solenoid valve.
Maybe a silly question but hopefully you will put me right, would a car or motorbike fuel pump be up to it?
There are three probes, two at the top, one at the bottom, the controller stops the pump when empty or full and actuates the valve. The Shurflo nautilus pumps don't like being run dry. With a sealed tank or a bag, you don't need all these electronics, which simplifies things, and reduces cost. Another advantage is that you're not compressing humid air into the dry spaces. A sealed tank will need to be bigger than the equivalent on an OTW boat, as the latter can use about 90% of the volume of the tank, whereas the former generally about 70%, a bag can typically pump to the capacity allowed it by a constraining jacket.
Fuel pumps aren't any good really. They make good pressure, but they're designed to pump fuel, not water, so the internals tend to corrode if you use them in submarines.
If you have the time, tools and talent, building a dive module needn't be an expensive process. The cost of components for a water pump system is typically under £30, the main expense being the pump itself. The rest of the module is just sheet material and the tubing. You can save a bundle by buying offcuts, or using salvaged materials.
PVC tubing is much cheaper than acrylic- often free if you keep an eye on builders skips- and much tougher, but is typically filled, so opaque. Most people like the concept of a transparent cylinder so they can see all the stuff working, which means acrylic, polycarbonate or unfilled PVC. In the UK the latter two are expensive and more difficult to source, which is why acrylic is more widely used.
