Simon,
You would be most wise to check the thread about the Sports Exhaust. In summary, and to answer your questions, the switch (if you get one, which, if you have the Sports Chrono pack, you don't) is low down on the centre consule (it has a picture of the back of the exhaust pipes on it), it defaults to off when switching the ignition off, no issue with insurance, cost to retro fit is much higher, increases noise only (I have seen no evidence of anything to the contrary and I bet it would be unnoticeable if there were to be a claimed slight increase in the bhp), it is road legal as it is supposed to not actually be effective at the 30-40 mph range- this is obviously somewhat dependent on your choice of gear as the system works on a rev range, not the car's velocity.
Mine? C4S, but I won't criticise a C2S choice. Black, which is, apparently, safe and dull. In my opinion black is beautiful- get yourself down to a clean one to see for yourself. I also happen to think that silver is as good looking, but, as has been mentioned, half of all cars on the road seem to be silver.
New or Used Approved? With a C2S you may be slightly better off than with a C4S, as it is further into its shelf-life. However, OPCs make money at every opportunity and, essentially, the UA cars are priced on the basis that someone can't wait and will pay nearly new money for something 12 months/9K old. Please remember what the OPC offers you for your UA car in 12/24/36 months time bears NO RELATION to what they re-sell it for. There is only one reason why Porsche are the worlds most profitable car manufacturer and this is it!
Get sat nav and phone, even if you, like me, will never even use it. Otherwise you will lose even more upon re-sell.
Everything else is individual.
Try Gordon McConnell, who is a buyer for Porsche, if you want the real-world lowdown on re-sale. His details are in the Porsche section of the Driving section of the Sunday Times.
I happen to think that my C4S is fantastic in every way (and I had a 996 C4S before to make some comparison) and I am yet to get it on the track, which I will be rectifying soon.
Good luck,
Derek