I started to buy N100 to N400 cars but I stopped quickly. The game gives you 1 to 3 cars per day if you play a little in sport mode and campaign mode.
Now, I'm winning N100 to N400 cars that I already have :-(... So, don't buy anything. Keep your credits till you finish the campaign mode with all gold medals.
But, if you really want to buy cars, I advise the Group 4 Ferrari 458 and the Group 3 Jaguar, or........... any GT-R as they are a little "cheated".