Your main problem is going to be finding the right controllers that can decode the RC servo pulses and drive the motors.
For the main motors, a Trex controller from SparkFun or a RoboClaw 2x5a controller from Orion or a Sabretooth controller from Dimension will all work, and will take two channels of RC. These can typically be set to do "blend mode steering" or "individual steering.."
For the auxiliary forward/back motor, another controller of the same kind you use for the main motors would work, but might be overkill.
For the "turn on only," you could perhaps get away with something cheaper, or just use yet another one of those motor controllers, as they can drive two channels separately.
For the auxiliary, and turn-on-only motors, you could also go with an Arduino-based setup, with one full motor controller (H bridge) and two power N-channel MOSFETs with logic-level gates as low-side switches (IRLB8721 for example, available in singles from Adafruit.)