Consider, too, how you want to construct the well so that the sampling personnel do not have an excessively long purge time. If these are shallow probes, you can get away with just a small diameter (1") probe. If these are deeper, you may want to consider the installation of some smaller tubing inside the probe that terminates at the screen depth and is connected to a petcock at the top of the probe. In a deep probe situation, you have a lot of stagnant headspace and ideally that would need to get purged to ensure an accurate reading at the screened interval. Depending on the pump rate of the instrument, that could take a while.
If you are near the water table, you want to ensure that the screen is long enough to not get flooded out due to seasonal water table fluctuation. If that happens, you will be monitoring the headspace above the water and not the unsaturated zone you are trying to.