Depends how practical you want it to be.
If you need the rear seats then it has to be a 911 in some form but if not the Cayman is actually a more practical car for 2 people. There's far more storage space in it and what there is is also much easier to get to than on a 911.
I thought my Cayman was a great car and I probably regret getting rid of it for the Exige and I'd definitely have another if it weren't for the lack of extra seats. The only worry about them is they do have a reputation for engine problems as do the 996/997s unless you look at any of the GT/Turbo ones so a Porsche warranty would be very worth having. Prices for the warranty are on the Porsche website but it's around £800-1000 for a Cayman/997 assuming it has FPSH.
You'd easily get a gen 2 Cayman for under £30k which means lower tax and better mpg. If I was buying a Cayman though I think I'd be tempted by a £20kish pre-March 2006 car with as low miles as possible to get the low tax and have less of a resale problem after doing 10k miles. The cars will take the miles but you will pay a hefty price in depreciation if you've even got anything like average mileage (10k pa) on it.
You could also look at any 996 (including turbos) and you should be able to get a 997 C2S on a 2006/07 plate (possibly with warranty) too.
I also wouldn't discount the Boxster but I'd ignore the Spyder for a daily drive.
993 is obviously another option. The biggest hurdle is finding a good one but there are good ones from £20k ish upwards. Sub-£30k you'd be looking at a C2/C4 as the wide body S models go for around £10k more than an equivalent Carrera. It's quite different from a Cayman/997 though and I'd keep aside a big chunk of your budget as a maintenance fund if you went down that route. They do look great though and much rarer than newer cars. Welcome to come and look at mine if you were considering one. Mine has probably had more weekend use already than the Exige did because of the rear seats allowing me to put Ethan in the back.
If you do go Cayman/997 try and get one with the PSE fitted, it sounds superb
2+2 seats, if everything pans out the 997T might fall in to this budget v soon.
