The WMR918 is an older version of the current model which is a WMR928NX, it has lower power solar panels on its transmitters.
As for fixings, you can buy poles from B+Q Warehouse. They sell them and brackets for TV use and they will do fine for a weather station.
Location for the sensors is:
Rain gauge, on a flat surface as far away from tall, surrounding objects as possible to prevent a "rain shadow" when the wind is blowing.
Thermometer, in the shade at all times. Also it needs to be away from any heat sources such as a wooden fence that may warm up on the other side and then reradiate heat.
Anemometer, as clear as you can from any nearby tall objects that will shelter it from the wind. Usually this means on a pole well above the roof line of the house. Often a local aerial installer will put this up for you.