In the states the lack of AC would be a major issue regarding resale value, in Notts (UK I'm assuming) it would depend on the ratio of cars with / without and the demand for it in cars. Cruise is also popular in the states (I never personally use it) but I doubt that would be of major impact on resale for this type of vehicle. On upgrading the stereo, there are plenty if DIY articles on it like this one:
http://986forum.com/forums/diy-project-guides/36276-dual-din-7-touch-screen-radio-install.html
It is an upgrade I performed on my car that has proven well worth it, Bluetooth / hands free calling is very nice and worth every bit of what it costs.
IMO the best way to ensure good resale is to keep the car up to date on it's service, keep any and all records organized in a binder specifically for the car, beyond that you are at the mercy of the market. If you do make the purchase, this forum is invaluable for help and knowledge, really a great group of guys here. I would also recommend these two books, they will pay for themselves quickly if utilized:
Porsche Boxster, Boxster S Service Manual: 1997-2004: Bentley Publishers: 9780837616452: Amazon.com: Books
101 Projects for Your Porsche Boxster (Motorbooks Workshop): Wayne R. Dempsey: 9780760335543: Amazon.com: Books