First, there's no good way to do this on the PS2. You'd have to either choose from limited pieces or take up lots of mem card space (and probably hook it up to the computer in the meantime). I have Need For Speed 3 on the computer, where I made Kurt Johnson's pro stock car...downloaded someone's Camaro, pulled out the wheel wells, stretched the front, added the flat spoiler, widened the wheels...wrote a torque curve, set the RPM range (min to redline) to 4500-10000, made it hard to steer. That's creating a car. I would love to do that though...make anything from my own 100hp 1991 minivan to a McLaren F1.
There would have to be inherent tradeoffs and limits to keep you from making a ten-ton Mini with 100,000 hp and perfect steering. Maybe raise the center of gravity...in GT3 it's about three feet underground for each car.
Same deal with creating courses. I have never played a racing game where that's possible. If I could, I would make a course from my house to my high school and take the highway back...but just think about all the things to add. Trees, parked cars, walls, signs, lights, billboards, start line, checkpoints, hills, road materials, bump strips, jumps, pit, autodrive pit instructions.
So, I would love both of them, but they're both impossible (except for making a car out of preset pieces).