Project goal: Turn on fan/motor when ambient temperature is greater than 25 C.
Concept:
Prototype
A shield with temperature sensor and motor driver can be used in conjunction with a development board to create an initial prototype.
At this point, faster development (fewer parts, surface mount assembly, easy programmability) are more important than accuracy, part life cycle, cost.
Temperature sensors: Several types of temperature sensors are available analog (LM335DT outputs voltage proportional to temperature 10 mV/K), digital with different interfaces (TMP144YFFR uses UART, TMP1075DSGR uses I2C).
Motors options will be looked at next. DC, Servo or Stepper motor. Neither high accuracy not high torque are important. Simplicity of hardware and software implementation are more important for this prototype.