I assume your designing planar arrays? If so, I'd recommend trying out Zeland Software's method-of-moments EM simulator IE3D. The software will display current distributions, radiation patterns and give you the antenna properties. They allow one to get an evaluation copy of the software which is nice.
As far as your second question you simply need to reference the Friis Transmission equation: http://en.wikipedia.org/wiki/Friis_Transmission_Equation
. You said you already know the Pt, the nessecary Pr, and the wavelength, therefore once you've designed your arrays and determined their effective gains you will know the maximum range of the system.