Interesting description you have there for this project. Here are a few pointers to help you out.
First, make sure your car model is closed . the one right now is open from underneath. Please also try to simplify it by merging the parts and making sure you have only ‘solids’ and not shells. As a reference see this model https://www.simscale.com/projects/Milad_Mafi/aerodynamics_analysis_of_a_sports_car_1
Further, as you intend to do a ‘Water Wading’ multiphase simulation, it would be necessary to have 2 inlets ( air and water) for the domain. For that you will also have to model the surrounding box with the inlet face split. For more details see this project. https://www.simscale.com/projects/sjoshi/wake_development_behind_a_boat
I hope it helps.