Each Chassis will have its own set of bodies that work with it, however I'll try to make each body I model work with as many different chassis as possible (by making a longer version etc.)
You'd be able to put a sedan, a coupe, a wagon etc. all on the same body.
We looked into it and found out that almost all cars of a similar class are a very simlilar size in all dimensions, for example a Mercedies S class and a BMW 7 series are very close to the same size, as is for example a Civic and a Mazda 3 - so what you'll have is a selection of different sized chassis, representing these major types of car (with different varients to fit mid engine, rear engine etc.)
Also - completly off topic, you're from Canberra eh? I only moved out of Canberra about a year ago, also my old housemate had an MR2 Turbo - fun car! (you should post about it in here -
viewforum.php?f=21)