What is the simpliest and cheapest way to control the motors
With an On/Off switch.
Sorry, I couldn't resist.
First, there have been other posts asking this question. Some searching of the forum will find them for a wider discussion and details on how to do this.
Break the project into parts and get each part working.
1- Get Ardiuno code to turn on an LED on and off.
2- Get Ardiuno code to turn on/off one motor through a Sabertooth module.
3- Get Ardiuno code to change the motor speed.
4- Get Ardiuno code to control all four motors.
5- Get Ardiuno code to send and receive data through its UART to/from your PC (do use a MAX232 or equivalent IC).
6- Design a data command structure to send to the Adriuno from the PC the control the Bot.
7- Last is replacing the hardwired serial communication with Series 1 XBees running 802.15.4 in transparent mode (AT command mode).