Most RC cars are about as hackable as it gets already. You can drop an Arduino in between the receiver and the motor controller/servos and drive them however you want with simple PWM signals. 3x3 feet is enormous, though -- you could try looking for sites that sell BattleBots starter frames, depending on the terrain you want to tackle. Here's one: http://www.battlekits.com/

How about powerwheels cars?

They're cheap second hand, they have a cheap plastic body so you won't feel back chopping it up to fit hardware onto it... They don't have electric steering, so you'd need a servo to operate that. However they're pretty good otherwise.

Edit: An article on the subject. Brief but you get the idea https://devpost.com/software/self-driving-powerwheels

That seems like a very good idea. I got one secondhand (given away) a few years ago for my little one and in the process of hunting down some replacement wheels and a new battery for it I stumbled into a rich world of powerwheels enthusiasts who optimize them for kids or for themselves.

The computerized hackability may be a little outside of that community but you'd have some great shoulders to stand on as far as the equipment.

It's relatively straight forward to build RC cars through hobby components, i'm not sure about a 3x3x3 car kit though - I think most kits target a different size. You can probably use the same sortof stuff to build such a thing though.

Look at hobby king for cheap bits...

There's also several published self driving RC car projects including code but right now my google is failing me, theres a couple hobby ones using rpi and such and one from a university I wish I could find.

found it, from georgia tech

- http://autorally.github.io/ - https://www.youtube.com/channel/UCSt0P1uqi4zU5RX2DZC_Qvg/vid... - https://github.com/AutoRally/autorally

https://diyrobocars.com, http://www.donkeycar.com.

Also, think about what you want. I see a bit of conflict between your two statements "an off the shelf car kit, which will let me focus on the software side" and "Extensible: I can add radar, etc. if needed"

>made to be hacked.

An oxymoron if I ever saw one.

I don't think there's any way not to get into the hardware part if you want to do something like this. If you are really opposed to hardware then might I suggest looking at some road traffic simulators and trying to simulate a self driving car? Might be a good place to start.

Intel's Euclid might be interesting for an all-in-one sensing and compute unit. Just need to interface with motor controller and steering servo. Arduino would be a good pairing for that. The RGBD camera is probably indoor only for depth.

https://click.intel.com/realsense.html

If you can't find one which fits your size requirements, i would recommend getting a modmypie rc car kit and using ROS to communicate b/w your laptop and the car.

On the software side you can hack around with PX4, a drone autopilot firmware running on Pixhawk board. PX4 is supporting rovers aka rc car. The community is great also.

Personally, any hobby-grade rc car that uses servos for steering, a real receiver, etc is pretty easy to hack.

For something large, I would say look at the Traxxas X-Maxx.

just received this as a gift, https://www.elegoo.com/product/elegoo-uno-project-upgraded-s...

haven't put it together yet, but its got everything you want except video camera, no reason it can't be added -- its just an arduino..

That looks really nice. How big is it? And is the software it's running open source, and made to be extended?

well, except for "holding a laptop", which is probably a requirement you'd be better without, What do you need screen, speakers, and keyboard on a moving RC car for?

something small like an Intel Compute Stick has nearly all the power of a laptop, but is very small and you can use remote desktop.

Sounds like you want a roomba...

http://spectrum.ieee.org/automaton/robotics/diy/irobot-annou...