Kee, excellent thread. Gets the old grey matter working this one.
The world is your oyster with regards to this one, really it is going to be down your personal preference. Looking at your choice so far of a Caterham, I guess you want something quite raw and interactive.
The Caterham will always be a good choice, be it in a value for money sense or from a driving experience sense, and this is why they remain so popular. Depending on which one you go for they are cheap to run - unless you decide to mod them in which case the Sky's the limit - and easy to work on.
A couple of years ago I bought a Westfield, working on exactly the same basis as you - and hated the thing! I never gelled with the car, and to this day I can't quite put my finger on why. Just one of those things I guess, but it is worth considering in your case.
Other options for you are either to buy a ready built road worthy track day car - or to build your own (but I don't know how you feel about doing the work).
I ended up going for the second option and built a Porsche 944 track car. In reality it was nothing more than a series 1 944 with everything stripped from it that wasn't necessary and a roll cage bolted in. I even ran it on standard size wheels and part worn as-cheap-as-possible tyres.
It was blinding! It lapped Brands Hatch slightly quicker than a standard Elise on standard tyres and did so all day long. All for two grand. This isn't meant to sound complacent but if I had crunched it on track, I could live with the loss financially. I didn't and ended up selling it for a profit.
This is something you could do with what ever car takes your fancy, and still hopefully fulfils all your requirements.
Best of luck.
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http://TegTypeR.blog.co.uk