In typical dry climates a correctly designed and installed evaporative cooler will save up to 80% of your cooling energy use. It will in most years give comfort for all but a few hours per day on a few days per year. These days will be the most humid days of the year. They will vary from year to year and vary by how humid your climate is. By contrast most rerigerative systems will also give discomfort for a similar amount of time, but this will be on hot dry days. On very hot days undersized refrigerative systems are prone to trip out, while evaporative coolers are more likely to keep going. In the more humid areas the balance shifts more in favour of refrigerative cooling. For more information see The Advantages Of Evaporative Cooling or How An Evaporative Cooler Works.
In general it is difficult to run evaporative cooling and gas heating through the same ducts. the main problem is that hot air coming out at high level from a duct large enough to provide evaporative cooling is moving too slowly to penetrate to floor level, resulting in bad temperature stratification (ie cold feet and a hot face). In fact one of the most common problems for reverse cycle heating is this temperature stratification, and this occurs with ducts intermediate in size between gas and evaporative cooling. In conclusion I do not recommend using common ducts for gas heating and evaporative cooling.
In most locations this doesn't happen, unless the cooler is in poor condition. One exception is coolers located at ground level rather than on the roof. For this reason, at least one cooler (the COOLMAX CM50) comes with an internal insect screen.
I think that most manufacturers have looked at designing a cooler for the attic or roof space, but because you can't take the air directly from the roof space (it is too hot), it becomes physically larger and hence physically more difficult to fit into the roof space. This also translates into a higher cost. It is a good idea waiting for someone to work out the solution. In the meantime for medium sized areas a wall or window cooler is a good solution.
In Australia it can be ordered from most air conditioning dealers. For commercial quantities visit the manufacturer's website (Munters).
A thicker pad allows either a higher pad velocity or higher cooler efficiency, but it has the problem of a higher pressure drop which reduces the energy saving benefits. Unless the cooler needs to mate with another device which has the same velocity, the size of the cooler is actually larger than a conventional 3 or 4 sided cooler.
Changing the temperature of the water supply will change the leaving air temperature, but only slightly. Basically there is a lot more energy absorbed in evaporating water than raising its temperature a degree (or even 10). To change the temperature of the air significantly would require a lot of refrigeration. A better approach would be to use an indirect cooler, however the advantage is still very small.
If you can't find an answer here try our Evaporative cooling weblog.