I know it's an obvious answer but you could do a lot worse than buying a cheap MX-5. You don't need to spend £4000 - get a cheap one and spend the rest on good tyres, maintenance and driver training. If you want something a bit different, look at a third-generation Toyota MR2. That's what I bought in the same situation. Neither will go expensively wrong and both will be more rewarding than a front-wheel-drive hot hatch on track.