gambleress, it will take about 27154 gallons of water to irrigate one acre the equivalent of one inch of rainfall. Each head will have a gpm delivery rate and from this you can determine how many heads you will need, to run each zone for say, half an hour. The total gpm of all the heads on a zone should not be more than the 60% of the gpm available at the water source (pump or Municipal). The 60% will vary depending upon the piping sizes and length of run; the number of heads will vary depending upon the shape of the area and the overlap designed in the system. Most heads come provided with a number of nozzles of different output. This is necessary in order to match the output (the gallons of water over a given area) of a quarter circle head to a full circle head on the same zone. These are some of the design features which you can decide upon and these will help you to choose an installer. Other variables will be material and labor costs. Hope this helps on what looks like an expensive project.