[Perfectlap]

1. ZERO accidents
2. get an S
3. lowest miles
3. 2000 or newer
4. buy from someone who had more than one car and only used it as a toy.
5. make sure it has HID (aka Litronics) and 18" wheels if you don't want to spend $2400 or more on new wheels and tires.

Try autotrader, Porsche Club of America website, Dupont Registry.
p.s.
don't buy black unless you like driving a car that is dirty half the time.