Ok so you could buy the RS5 if you want (RS5 costs 150k thus being the most expensive A5 car) then for A4 I suggest having the Jaguar XKR Speed and Black Pack, Caterham RS500 Superlight, and Ford Shelby GT500. Altogether it costs 300-400k but it is worth keeping in your garage.
TBH before I updated the game I had to exploit the Cup Glitch where I can repeat a cup race then lose it but still end up winning 1 million, 2.5 million, or 5 million.
BTW, I decided to make a suggested car list for A5-A1, B3, and C3 races.
C3- Dodge Charger R/T, Jaguar D-Type, and Ferrari 308
B3- Spyker D8 Peking to Paris (if you have the money) and Mercedes ML63.
A5- Audi RS5 and TT-RS Roadster
A4- Jaguar XKR Speed and Black Pack, Caterham RS500 Superlight, Ford Shelby GT500, and Aston Martin DB9
A3- Aston Martin DBS Carbon Black Edition and Ferrari 612 Sessanta
A2 (this is going to be a lot since A2 is my favorite class)- Aston Martin V12 Vantage Carbon Black Edition, Ferrari 458 Italia (if you're playing on the PS3), Chevrolet Corvette ZR1, Dodge Viper SRT-10 ACR, Mercedes Benz SL65 Black Series, Mercedes Benz SLS AMG, Ford GT, Audi R8 V10, Ascari KZ1-R Edition, and Ferrari 430 Scuderia
A1- Bugatti Veyron Super Sport, Koenigsegg CCXR Edition, Ferrari 599XX, and Pagani Zonda Tricolore
So those are my suggested cars for each class in Hawaii.