Saying the engine and gearbox are the only things unique to the IS-F model is at best plainly wrong, LSD, wheels, suspension parts, pumps and other ancillaries are model specific, yes a radiator is not a moving part but to say it cant "rust" is not completely true, alloys corrode and therefore can leak.
Nobody is saying an IS-F is a totally bespoke car, they are sayng how reliable it is.