what arduino gear should I get?
Let's get some questions answered first
through Bluetooth (smartphone app, iOS & android)
Do you really need both? Using Android and Classic Bluetooth v2.0 is the lowest cost and most simple. Unfortunately you can't easily use it with iOS. If iOS is a must, then you will have to use BLE (Bluetooth 4.0). This can be used with Android. BLE can be complicted when using the lowest cost modules, and an integrated solution with good support like the Adafruit Feather might ease the job.
For KISS, stick with Android and an HC05 module.
How do you plan to house/package the arduino and Bluetooth module. Battery or wired power? Is this an industrial environment? Vibration? You will probably need the Arduino and Module soldered on some sort of perf board or expansion shield instead of a solderless breadboard. A Nano might be a good choice for this.
PWM Regulation range:5-100%.
Is the speed control PWM at 12V? If so, the Arduino 5v pwm output will have to drive a transistor.
Are you planning any speed feedback with an encoder? What are these motors going to be doing?
You will probably need some sort of relay board to turn the power to the motors on and off.
What Arduino experience do you have? What motor control experience?