If you want to go for a 996, go for a facelift car. You can recognize the facelifted cars pretty easy, they all have the Turbo/GT2/C4S/Targa frontlights and 20 HP more power (320 HP or 345 HP with Porsche powerkit).
I cannot recommend buying a 996 before MY 2000 but this is just based on personal experience and the experience of owners I know.

Caution: the new 911, the 997, is soon available worldwide.
As soon as the 997 hits the streets, 996 prices will start to drop and I expect a pretty nice batch of used 996 to be available next year from most dealers worldwide at attractive prices. So don't rush in buying a 996 now, wait if you can.

I also want to give potential C4S/C4S Cab buyers a personal advice: think very well about your buying decision because the C4S/C4S Cab is based on "old" 996 technology and the 997 is definetely the better buy right now, especially the 997 Carrera S. The 997 Cab will be available spring (Germany) next year. You don't have to listen but also don't say I didn't "warn" you.

Don't forget that the 997 is much wider than the standard 996 and almost close to C4S width.