Thiose numbers sound like the thickness in inches. thie first one is about 3/32", and the second is a little under 1/4".
the thicknes of the insulation depends on what you want.
Thicker insulation will provide cooler temperatures. At a bare minimum you want enough insulation to keep consation from forming on the outside of the cooler. To figure out how much insulation you need for this, you need to learn about heat transfer. Just read up on one dimentional heat transfer, and find the thermal resistance values of the plexi and styrofoam, as well as the water temperature and air temperature. From this and some other approximations you can find the heat loss, external wall temperature, andything you will need.
At a first glance, unless you are getting your water very cold, like -20 or more, the 3-4 inch should prevent condensation.
I would be worried about the plexiglass cracking or breaking, especially if you are cooling it a lot. Since you are insulating it there is no need for it to be clear, so just use something stronger and cheaper. A simple wooden box with a liner or something like that will work nicely.