Check your prize cars often as those cars usually fit in the regulations and have better handling than a tuned up car. For example, try the following cars---all prize cars from earlier races:
150 hp: Porsche 914 200 hp: VIS Integra (lowered to lowest, upgrades for acceleration and brakes) 250 hp: Mugen Civic 300 hp: Mugen S2000 350 hp: Kaira Impreza (M20 b) 400 hp: Kaira Skyline 450 hp: Mazda Amemiya RX-7 Big Block Shoot-Out: Tiger Corvette Corvette race: Guldstrand Corvette (lowered to lowest and upgraded to U999) Impreza vs Evo: Sparco Evo VIII Great American Face-Off: Shelby Mustang GT 500 5x5 Supersprint: Magellan Financial Viper Light Weight: Ferrari 330 P4
If you do not have a Ferrari 330 P4, buy a Ferrari 430 and upgrade the following: racing brakes, racing tires, racing springs and last weight reduction. You can also upgrade the engine if desired. Reach level 25. The early races are easy wins and give a small amount of money, but you can get more money in later races. Go to the Manufacturer club races and win Club Della Scuderia Ferrari to get a Ferrari 330 P4. You can also buy it for 235,500 credits if you have the extra money.
To get the classic Ford GT40 and upgrade it, you first need a Ford Focus SVT for 9,500 credits. Paint and decorate it (may not be necessary for Reputation levels). In the Factory spec races, win all three Focus races. When you get the message "There are new Ford Supercars available" buy the Ford GT40 for 235,500 credits. The Ford GT40 is a classic racecar and thus will need handling improvements. The new Ford GT has better top speed and handling if you just want a supercar. Buy the following: race upgrades, wing, wider tires and weight reduction-- without getting to U999 class. Changing the engine or tire compound will take it to U999. This car is perfect when doing early races again. It fits into Proving Grounds (RWD, Midengine and Flyweight) and Amateur (Sports Car, Classic, Freebreathing, Big Block Shootout and 20th Century). It is also fun to run in the 20th Century and beat top modern supercars with a 40 year old car. It also goes into less than 500, 600 and 700 hp.
The VellSide Toyota Supra is an impressive car. Its handling is below average so upgrade any areas that improve handling-- weight reduction (last), a wing, racing brakes, racing tires, racing suspension, stiff roll-bars, etc. Also try buying a new transmission/clutch/driveline to handle all the power.