I'm a Porsche fan too, David (xtr2turbo) is right to warn about the engines though, you just need to do a lot of research and buy right. I think you're in a sensible thought process and at the end of the day it only has to be the right decision for you.
Marto and I have just bought a cheap Mazda mx5 - MK1 1.6 - including the cost of a cage, bucket seats, alloy wheels and tyres, new rear brake calipers and pads all around, we're into a total cost of £270 each. Just did a trackday at Bedford on Saturday and it was the most fun I've ever had, such a rewarding driver's car. If I were you, I'd be tempted to take a punt on a very cheap one on eBay with a little MOT remaining, go and hoon it about for a while and then make a decision. You won't be scratching that Porsche itch but you might save yourself a small fortune.