If you insist on going for classic insurance, try an old MK1/2 Golf instead. A rear wheel drive sports car just isn't going to work.
I don't know who's told you that any 15 year old car will be really cheap to insure but they're clearly wrong. I expect a McLaren F1 would cost more than £500 to insure for a 17 year old, for example.
Unless you find a reliable old banger that you can put on a classic car policy at such a young age with zero no claims bonus and using it as your main car, you're going to have to spend at least £1000 on insurance*. Just remember that, as someone said earlier, many classic car policies don't attract no claims bonus that you can apply to modern cars so you may be shooting yourself in the foot in the long run. her0n's age and driving experience (and presumably solid NCB) is how you get cheaper premiums.
EDIT: *unless you sign up to one of those smartbox policies which limits when you can drive and tracks how well you drive.