I'm not the developer, but AFAIK, there will be car designer, which will contain all these design aspects, excluding engine. And, citing zeussy:
The flow of the car creator will be similar to the following:
Chassis Type (Ladder, Monoque etc)
Car Base Size (Length, Width)
Wheel Base
Suspension Type, Brakes
Engine, Placement and drive type (FWD, AWD, RWR)
Seat counts and placement
Cargo area size and placement. (Long flat to make sedan, tall short to a make hatch)
Body Choice, from a large set of prefabs, these will have no headlights, handles, fixtures etc. These bodies will have handles on them to manipulate them.
Fixtures (headlights, handles, bumpers, wheels etc) to make your car unique.
What is not mentioned in this citation, is the gearbox choosing, which also will be in game.
Internal components other than engine will have simpler designer, just choosing some preset (like 4-speed manual with 2 first gears synchromeshed, in case of gearboxes) and changing some values.
Work on car creator isn't even started.
But, as I mentioned, I'm not a developer, they'll explain it better, and propably more correctly.