I would hate to say, Use real car physics. There is a car in the standard assets that you could use as a base. It would be a cross between AI and user controlled I am guessing, and you would have to get the desired AI direction from the spline. If the player goes to fast, then he would spin out around corners. The AI would then try to compensate by trying to steer back towards the last waypoint.
As far as the spline system, you could buy it, but you will still have to convert that spline into waypoints. It isn’t very hard to create splines 2d or 3d.
You could check out MegaShapes, it has a couple of components in it for moving rigid bodies and objects along splines, even comes with a car driving around a short track example, it might do what you need or with a little scripting, I already use the spline system in it for controlling my cars along spline paths, very easy just use the API to find the point on the curve and apply a steering angle to the car system to steer towards it.