It depends on your goals for the car the best mod you should do.
Yes 350z springs "bolt on" to the G35 chassis. iirc it is about a 1" drop that is gained with 350z factory springs.
Tein makes/(made?) lowering springs for the v35 chassis S techs and H techs both are good for a nice drop in height without sacrificing ride quality.
However, coilovers for this car can either be in two forms true coilover where the factory spring bucket is eliminated (this will require you to purchase adjustable rear control arms and toe arms to replace the spring bucket, but this combination gives you full adjustability as the vehicle goes lower.
The factory style coilover (separate spring and struts) are also good yet when lowering the vehicle the factory alignment bolts will not be able to bring your car into proper alignment specs (after a 2 inch drop iirc)
The best coilovers/most popular for this chassis tend to be BC coilovers, they're better bang for your buck and are decent in ride quality. KW also makes a good offering in addition to Fortune Auto.. Keep your eyes peeled and search this forum and other Z forums as there is quite a bit of good information out there already