I accomplished this in the year 1965 in career mode. First thing I did after completing 1964, and beginning 1965, was purchase the Granatelli (Turbine) Chassis. This chassis is rather expensive, breaking the $200,000 mark. However, it does not require you to purchase an engine or gearbox, as it is a complete car. If you bought conservatively over your first four seasons, you will have enough to pick this up.
A word or warning though! This car is EXTREMELY different from any normal combustion-driven car you have driven. It runs slightly slower, and handles a lot better than the combustion cars. I advise using it at Trenton for your first race, using it at the Indy 500, and buying a new car after Indy. I used the car through Milwaukee, and it was horrid; I was getting Pole Position every race, I got 11th, and finished 3rd overall that race. Combustion cars are definitely the better choice right now.