Agree with Dean. Look at and drive as many cars as possible. Depending on where you live, try to link up with one of the regional Cobra Groups to learn about the different makes.
When you find a car that you might want to buy, post in the regional club forum where the car is located. If you're lucky, someone on Club Cobra will know the car and can vouch for it.
You can't miss with a Superformance.
There are a lot of great cars and deals out there. Good luck in your search!