Batracer Constructors - looks promising
 

Batracer added new content - it's paid and not cheapest but it's one time payment only - I'm really considering getting it.
Basically main idea of this new update is that you can fully develop your car.
More info here http://batracer.com/-uk?constructors
And official youtube video http://youtu.be/JT-5L-Y0FNA