How much water you lose involves too many variables to predict. The ambient humidity, system specifics, and how you've got your fan set up all factor in. Ultimately, the more water you lose, the more effective your cooling will be since it's evaporation that accomplishes the cooling.
With your system, I'd recommend looking at the largest chiller you can afford, up to about 1/4hp. The reason why is that chillers are expensive, but going up in sizes is only an incremental cost, so by oversizing the chiller, you are paying a bit more now, but if the time comes for you to upgrade tanks, you aren't having to buy the entire unit all over again. I bought a 1/5 for my 55g reef, and when I upgraded tanks, I had to drop $1000 all over again for a new chiller, where I could have spent a couple hundred more up front and avoided that whole cost.
Of course, don't go crazy... larger than 1/4hp and you'll have trouble meeting the minimum flow rate for the unit, and it will short cycle, wearing on the chiller and causing rapid temperature swings.
