I want to turn a mini 5v fan on/off with an Arduino. It draws about 160mA current. If I power this via the Arduino Nano's Vin or 5v pin and control it with a transistor and perhaps even use PWM with 50% duty cycle, do you think I could safely control it without the need for an external PSU?
How will you be powering the Nano? Via USB?
USB can provide you with 500 mA max. So, 160 mA for the fan + a few 10s of mA for the Arduino should be fine. Mind you, this is while using the vin or 5v pins, NOT the I/O pins.