The thing to remember is that that the car will moving at less than walking speed, it does not need to go fast. The following is just from my experience ...
There are 4 parts each as important as the other to get right in building a rig;
1/ How its connected to the car.
2/ The boom or arm section.
3/ The head end, how to connect your camera.
4/ How they all fit together (the tricky bit)
There is 2 main ways of connecting to the car, an under-car kit, using magnets or clamps (good for minimal photoshop, bad for universal fitment and magnet reliability) and suction cups, which will fit 99% of cars but leaves a fair bit to photoshop out.
The boom arm needs to be a fine balance between rigidity, length, weight, and physical size, plenty of trial and error goes into this part alone.
To connect your camera most people use a Manfrotto magic arm combined with a super clamp as its plenty adjustable, and the clamp will fit over most pipe/boom/arm diameter ( I don't use one myself because I can't afford one yet, lol)
The really tricky part is how to connect your suction cups (if thats the method used) to the boom keeping it rigid but also allowing it to be adjustable for both reach and curvature of bonnets or glass etc, an engineering mind comes in handy.
This is just the start, there's then the whole lighting/scenes/photoshopping/camera settings/car sourcing/weather predictions/what to have for breakfast/etc/etc.
Enjoy.
Ps: don't spend loads of cash, if your having problems finding something, let me know and I'll have a think on how it can be done cheaper.
Pps: cheers Keltic