I'd come at this a different way, although Peter touched on it.
A decent S2 will easily use up your budget, particularly if you allow at least £1K of it for the first year's running costs. That'll give you a car that, bar chipping and a decat, is at it's most potent unless you supercharge it. Probably with, currently, a little over 200 BHP if it's in good fettle. To that, you will want to add decent suspension, upgrade the brakes, possible a set of track wheels and tyres, lighten it, add a cage, etc. You're now way over budget, by £000s. For that you could have bought a turbo, probably one of the really sorted track/road cars like Andrew Sweetenham's recent sale, or RPM's turbo sold last year, with 300BHP and more to come. I can't see how you'll do it unless you buy a cheap S2, which will need money spending on it anyway. Catch 22.
Why not buy a Lux? A really good one will eat up half your budget, and will be arguably more fun on track once you've spent the balance of the budget on it. You'll easily create a car that'll lap faster for £5K if you start with a Lux, I'd have thought, and they really are no slower in real driving conditions on British roads.
Or, save up another few £000 and buy a prepared turbo. That'll give you a very capable track car with half-decent road manners in a way that's been tried and tested to destruction by many owners. It's the only sensible way to do it on a budget IMO