I agree with Mike. The Tornado car is the best bang for the buck, but remember, the "basic" price is for a very "basic" car (no A/C, no leather, and a very low powered engine). I priced their car with options such as A/C and leather, and a better engine. Suddenly, the car became expensive.
The GT40 is not a car to be done cheaply. You would be disappointed in the long run.
I have spent a year researching the GT40, asking people like Mike Starling, Gordon Levy, Wayne Presley, et al, and have decided that to do it right, you are going to spend some money ($55K+). Anything less just won't do the car justice.
If you want a car less engine only, the CAV GT40 at $56K is great. Just pop in an engine and your good to go.
If you want to build one that will look great, and go like a rocket, go with RF or Tornado with the options.
If money is not a concern, ERA is an excellent choice. However, they have a 2 year waiting list.
Hope this helps
Bill D
Los Angeles, CA