If you already have another car and want a pure track toy then I'd suggest you go for something more extreme than a souped up road car. Light weight and tactile are what rule on the track (and dare i say it rear wheel drive).
You can't beat the Westfield for value for money, light weight, low wear and tear, feedback from the road, sheer, pure driving experience (Caterham if you have more cash).
If you really have to have something with a roof then I'd second the S1 Honda elise.
Anything less extreme than those is a compromise and if it's a dedicated track car then why compromise?