Thanks for the real mix of opinions guys. Really helpful and a bit of healthy debate is always good. Without meaning to sound like I'm ignoring advice and I am sure I would enjoy any of the suggested cars (MX5, MR2, etc), I'd still like to pursue the 350Z idea further.
To put things into perspective, it costs me a small fortune to keep the Z4M on the road (£460 tax, £400-900 for services etc). I live at home, with a high disposable income and cars are my hobby. I'm not mechanic, but my friend (post name - papercutout) is good with a spanner and we'd be working on it together. I just got a new Scirocco as a practical daily from work, so I only pay fuel which is about 45-55mpg, rather than 18mpg in the Z4M. I've maintained the Z4M regardless of cost for 2 years now and it is expensive, but I've had the cashflow to keep it looked after as a daily.
I appreciate that you can make a track car for a lot less with the MR2's etc, but the 350z is fun to drive and I would only be using it for a track day a month at most, possibly a trip to nurburgring and meets at most. I wouldn't be driving it daily either. I'm not looking to be the next stigg or set the fastest lap time in the world, but I am looking to have fun out there and deviate slightly from the norm..
Does this change anything or am I still mad to consider this little venture for fun..!
The cars I would consider, but won't limit myself too included:
350z
S2000
Classic Subaru
Evo
Cars which would make more sense and I'm not ruling out, but still aren't hugely exciting to me at the moment OR don't fit my requirements:
MR2
Westfield
BMW E36
VX220
Not sure if there are any others I should consider really.