There's 8 cars you can buy with the 18k credits it gives you.
The best car you can buy with that is the Sprinter Trueno. YOu might not want to buy it though because after you win the Sunday Cup you get a better version of it.
I'd suggest the Sprinter Trueno, but you might wanna get the PT Cruiser or something, it has more HP than the Trueno, but weighs more, and is more expensive...
Anyways, it's really up to you, and which car you like the most. You're going to get about 10 different answers on here...