The Altezza/IS200/IS300 and GS300/430 Mk2 brake pads and discs are the same.
The Altezza/IS200/IS300 lowering springs, shocks and coilovers will fit, but since the IS300 is quite a bit more front heavy, the drop in the front will be much more if IS200/Altezza springs were to be installed on an IS300. This diff. in drop may somewhat affect the ride and handling of the car.
Most coilovers were designed for the Altezza, but since these can be height (and sometimes stiffness) adjusted, they should fit nicely on all the engine types.