Ok, here's how to change the cars. look in the PVEH folder, and swap HIGH\US\Car1A.* (meaning all the files with the prefix Car1A in that folder) with any of the other cars in other folders (meaning if its Car1B, you have to rename it to Car1A). Since the pictures are posted above, the cars will be listed from from top to bottom according to the pictures.
\PVEH\MEDI\AS\Car1A.*
\PVEH\MEDI\US\Car1A.*
\PVEH\MEDI\US\Car2A.*
\PVEH\HIGH\US\Car1B.*
Cant get the yellow version of one of the cars tho. And this doesn't work with regular traffic cars unfortunatly (ya, i tried =P)
Also, if you open up Language\Strings\mainUS.bin in a hex editor like UltraEdit, you can pretty much replace any text that pops up when you're racing. its a little tedious, but its kinda funny to change words here and there. A common String would be 540041004B00450044004F0057004E which is for "TAKEDOWN" sooo, whatever. If you're too lazy to figure it out, and just wanna see it, try replaceing that last string with 5300550043004B0020004900540021 Which will make it say "SUCK IT!" instead of "TAKEDOWN"
Anyways, thats all i learned while messin with this crap. would still like to know how to change the number of laps. replacing the video files speeds things up, but just 1 continuous race would be cool. If only someone could shed some light on this =/