Help with arduino car project
1) start simple - adding extra features will not help you, especially if you have a deadline

2) implement and test code for each low level function separately - however, in case you use libraries and or snippets, make sure that if a library/snippet requires exclusive use of a certain feature (ex: a timer), others do not need it.

3) test the overall logic elsewhere, where it easier to debug (ex: on a PC), by replacing the actual function calls to low level actuators/sensors with mockup functions that do I/O through either a file or a console.

Said this, what do you mean that you didn't understand the examples you found? To understand most of them, you need at least to read the API/interface of the libraries they use. And also reading the reference manual of the ATMega328P for the HW blocks referred by each library will help getting an overall feeling of what the library does.

About the basic physical model: once you have 4 DC motors (but 2 plus a caster would be enough), why do you want to add a stepper? Youc could do all of it with just 2 stepper motors , which will give you a simpler way to control precisely the orientation of the robot.

I'd suggest you to get the basic movement sorted out, before worrying about the rest. Just make sure that it doesn't make exclusive use of the microcontroller.