Seems like saving money by picking a slower car doesn't work. Just checked and can only afford the Hyundai even though I saved 10k by picking the BMW where I could have taken the Citroen for Donington. Wally, so there is a reset after every race, right?!
That's right, you don't save any money round to round. You have to gamble a bit. You can choose to pick a fast car and win, bank some points, and then be in the Hyundai next round. It depends on who else is driving what too.