EasyIoT

Here is list of materials with buying links:http://iot-playground.com/blog/2-uncategorised/94-esp8266-smart-plant-irrigation-system#materials

hi, here you have more tutorials:http://iot-playground.com/build

yes, look at this link:http://iot-playground.com/blog/2-uncategorised/10-low-power-door-window-sensor

and this:http://iot-playground.com/blog/2-uncategorised/9-arduino-low-power-sensor

I guess yes. Use 3,3V relay.