I have owned a Porsche for the last 9 years.
Its a 1994 993 C2 (which is a 3.6L 280bhp, not 3.2L), which for those not in the know, is the last of the air/oil cooled 911s.
15k wil possibly get you one of these, but more likely to get a 964 (88 to 94), which has a 3.2L, and about 265bhp. The 964 is much more tail happy, and feel somewhat older to drive than the 993 (imo), as they have an older style of suspension and gearbox. Thats not to say they feel bad to drive, its just a personal feeling having driven a number of them over the years. The interior in a 993 is almost identical to the 964 though, which as Ali says is definitely old school.
The 993 is the one most of the collectors go for so they tend to hold their money quite well. Mine returns 24mpg for spirited driving, and gets down below 12 if continually driven hard on a track day
Servicing wise they are cheaper than the newer water cooled 996s for the annual service. There are many independants about too, so no need to go to a main dealer unless you really feel the need.
If your going to get a boxster, make sure you buy an S. The non S is a great car, but everyone I've known who has had one soon gets fed up with it as it seems under powered.
The Boxster S is a great track day car as it is very well balanced. It makes a great everyday car too. Steer clear of one with ceramic brakes as they are blindingly expensive to replace. For everyday driving a Boxster S is just as quick as a 993.
I've thought about going for a newer one, and the S is definitely one I consider as a good replacement for the 993.
If your less sold on the idea of a Porsche and can live with a little less creature comforts, I'd suggest having a drive in a lotus elise or exige. Totally different kettle of fish - very light and very agile, but just as quick point to point. Possibly need a little more attention though (can't say for sure as I've not owned one, its just a feeling based on comments from mates that have them).
If you need any more info just ask away. I've been a member of a couple of porsche clubs and forums for a number of years, so can point you at a couple if you want to do a bit more research.
Hope that helps.
John. (too many toys, not enough time).