Check out my project below for a examples of how an Arduino can be integrated to support various SmartThings “Capabilities”.
My “water sensor” device class utilizes an analog input, as does my “luminance sensor”. The “contact sensor” utilizes a digital input. This should get you started.
You could also take a look at my “Temperature/Humidity” class as it handles everything, however it is designed for a DHT series sensor. You could hack it to use 2 analog inputs instead.
Since your needs are simple, I would use an Arduino UNO R3 board. However, if you want more flexibility, get an Arduino MEGA 2560 board as it has 4x the RAM and three extra Hardware Serial UART ports. Either route, you’ll probably want my version of the ThingShield library as I have made optimizations and enhancements to it.