I've got a Cayman S at the moment that I'm about to get rid of after 2 months of ownership.
First off I'd aim for an S if you can, there's a big difference between the two and I seem to remember the 2.5/7 having issues with oil consumption. A non S also comes with a 5-speed box if thats an issue (it would be for me, the noise gets a bit much on long journeys and thats with a longer 6 speed box).
I'd also look at the performance per pound ratio. Handling wise my Cayman is awesome but straight line speed in my opinion is lacking. The 2.5 I had a go in was VERY slow for what it was. For the amount it costs in tyres, fuel, discs and pads its not actually that quick. To put it in perspective, my RS4 cost similar money to run but was leagues ahead in terms of pace. If its a weekend toy then maybe something a bit more special?
To be fair, those cheaper boxsters do look good vfm but I'd be wary of high mileage cars or ones that haven't been looked after. And find a decent Indy specialist. Theres no such thing as a cheap porsche.
You might also want to consider a 350z. Had one of these for a bit and it was much more fun to drive, LSD as standard and parts were 'a bit' cheaper. Similar money will get you a much newer car. If it's a soft top you're after, an S2000 (if you're not over 5ft11). 7k also gets you an M3 (E46) with similar running costs, an M5 (with astronomical running costs), probably an SLK (which would be rubbish) or....er...I dunno.
Not much help really, sorry!