For fleets it is common to use between 1500-2500 psi, more than that is not necessary. All the higher pressure will do is strip off the paint. Use the
nozzle chart to find your correct tip size. Go across the top to the pressure you want to achieve (not higher that the pumps rating) and go down the column until you reach the flow capacity of your machine. Then follow the column to the left to get your nozzle size.
For fleet work you really want 5 gpm minimum, 4 gpm will work but it takes a lot longer and time is money.