That's really strange, you ask for high requirements, lot of polys ,etc ...
and incredible you choose 3DGS Pro ???
BSP is slow compared to models, and it's an outdated technique.
why not choosing Torque engine , in combination with LOD you could have something a lot better and with top notch network power if you need network.
If some company would say to me to make a game like you target,
i would before try to find the right powerfull engine, and some dedicaced artists kwnowing well urban,circuit,cars environment on CG forums.
Not to talk too bad, but with a city like in your first picture, it can only be an internal project
not lot of people would play it with that simple copy and paste of bad textures.
I think you are dreaming