ok, cool.
So basically you just need to have one detector in front, two motors, and control the left motor whenever there is something in front. That way it will steer left until there is no longer something in front...
This actually shouldn't be too hard to do. It's nice and simple.
1. Look up how to detect an object in front of you-- I think your easiest approach will be to use an IR LED and IR remote control receiver (usually in a 3 pin package). Usually pages that talk about this describe how to modulate the IR LED at 40khz. I've prototyped this approach and it works pretty well.
2. Figure out how to use a single transistor to turn a motor on and off.
3. Figure out how to turn the transistor on/off based on output from the IR circuit. Hint: the IR receiver outputs pulses when something is detected, so you will need to use a capacitor, I suspect, drained through a resistor, to ensure when something is detected the voltage remains constant and high and when something isn't detected the voltage drops pretty fast.
Ok, do some research and see what you find...
Btw, logic chips to do all this is kind of overkill I think since you should be able to do it all with a few discrete components.