hey noggin, umm...I have the feeling I already know ya from somehwere :>)
Keep it simple as 123, no point in re-inventing the wheel. I would suggest that you get a hold of an old tank sender unit outta the scrap yard, the kind used to indicate your fuel level in the tank of a car. Use the rod mechanism and float etc with a small paddle of some sort on the end and 4 cheap optical sensors, you can always cut/bend the rod etc to achieve the correct angle and arc. The sensors would be arranged with some discreet logic to detect failure and either shut the pump off, of empty the tank, either way, it gives you an opportunity to indicate failure of the control mechanism. You will need to be fairly inventive in order to strip it down to something useful to your application though.
Use a bistable or something equally simplistic, or a small micro if you must, to control your pump. You don't really need to pump it out as such, just prevent it overfilling, so maybe a 20-30 sec burst of the pump will be sufficient if the pump isn't rated for the duty. If the pump is capable of safely running long enough to completely empty the tank then use something state driven, much less chance of a failure or screw up.
You could always use a small micro and just read the resistance from the rheostat on the tank sender, if you can find a way to keep it as part of the mechanical setup, and just perform a table read/compare to control the pump.
Also, make sure your tray is sloped slightly and draw the water off from the deepest point.
in the case of a submersible/ flooded suction pump, you'll want this point to always have enough water in it so as not to lose the prime.
In any case, the more complex it is, the harder it will be to build, the more expensive it will end up, and the less reliable it will become.
Just my thoughts...