I have a 1999 C2 Cab and I can recommend Michelin Pilot Sport front: 225/40 ZR18 and rear: 285/30 ZR18. Porsche optional wheels would be front: 7.5 later 8x18 ET50 and rear: 10x18 ET65. Ernie's Alloys,Inc. in Miami, Florida (305) 687-0028 has "Take-Offs" for very good prices and ships all over. I currently have Porsche 18 "Turbo Look" wheels and will be selling them to get a set of Porsche 18 "Sport Classic II" for "Street" driving and Porsche "Carrera" 5 spokes for the "Track/Autocross."
As for lowering your car I've found H&R springs might lower your car too much. PCA's "Tech" reported a fellow with a 996 that lowered his car below 25 inches (rough measurement from the centerline of the wheel, ground up to the bottom of the wheel well) for the street and hit a bump in the road and broke his engine case!
Ask what the final ride height will be with the H&R springs because they might lower the car too much. Typical ride height should not be lower than the Euro M030 "Sport" Chassis Porsche option. If you want larger tires/wheels than what comes on a Porsche standard coupe you might need to consider H&R coil over all around which help with the tire/wheel set-up like on a Porsche GT3, but all that's some big $$$! Best thing to do is stay with Porsche tire/wheel sizes without any problems. Porsche's own Euro M030 "Sport" suspension set-up is the best for the 996 without any problems; ment for the standard coupe suspension and tire combo.
Good luck!
Kent
1999 Carrera Cabriolet