Cheapest and Simplest way to build a LEGO Technic car?
I think your best bet is to start with 42048 Technic Race Kart and a 8293 Power Functions bundle. This gives you just one motor, but that's enough for propulsion (going back and forth).



You would need to look into a motor driver board (for example the DRV8835) for the motor, and look at schematics for a power functions cable mod to be able to drive the motor directly from the RPi - see this question for several suggestions on how to do this..



For steering you can look into a 88004 Servo Motor, or use a normal Medium motor with extremely fine control or a good set of gears to be able to make small steering adjustments.