I can't exactly remember what you can tune because my TV is sitting on the floor, but I can remember you can change tyre type, weight, gear ratio, et. I don't think you can change the engine though.
There's a number of points you've got to take into consideration. In S class you can only use tuned cars with a limited amount of points. For example, if 10 points was the limit (which wouldn't happen) and I took some weight off the car, I would get more points and could end up going over the limit, so I'd have to make some changes.