No the voltage will not be "stepped up". The ESC will probably work at a lower voltage but will not be able to give the rated current. So it's best to use a 4S battery if you want to use it at it's rated current.
Dagu is now selling a new kit that fits on top of the Rover 5 chassis: the service droid. I got an early version and just started playing with it. At the moment it's remote controlled by another Arduino through nRF24L01 modules. But I will add a raspberry pi and camera and try to do some autonomous stuff.
The technical details of the TFT show that digital pins 2, 3, 5, 6, 7 and analog pins 0-5 can still be used. I think the only way of connecting something to those pins is soldering wires to the pins on the bottom of the Arduino.
There is an Arduino modbus library: http://playground.arduino.cc/Code/ModbusMaster Only the Arduino Due (32bit Arm micro controller) has 2 analog outputs (12bit). The Due also has 12 analog inputs with 12 bit resolution. Other Arduinos (AVR based) only have pwm outputs and 10bit analog inputs. So you would need additional hardware to accomplish what you want. Bluetooth is pretty easy to accomplish, you can buy a bluetooth shield or just connect a bluetooth module to your Arduino. Programming them is pretty straight forward as it's just serial communication.
From my experience you will only get a range of 1km with 2 modules with PA+LNA . And you will need to use the slowest speed (250KBPS) and have a line of sight between the 2 modules. There is a reason they are cheap
Those motors can be used for a robot, but it will be a slow robot... I've done it
I was using a 3S Lipo (+-12V), makes the motors run pretty hot. I haven't killed them yet. They are dirt cheap so can take the risk I did need to write code to slowly ramp up the speed else I could not let them run that fast. And I used 2 74HC595 shift registers to control the 4 ULN2003 boards. So I only needed 3 pins to control the 4 steppers