Why are people so obsessed with mileage? Vast majority of modern cars, including 350z last for 200k+ miles without any major issues.
Your car is 11 years old. Whether it has 25k, 50k, or 100k miles on makes no difference to reliability whatsoever. It all depends on how was the car driven and maintained. If anything, at that age you should be concerned about mileage being low rather than high.
Paying to change a car that has done 70k miles for exactly the same car that has done 40k miles is madness. There are much better ways to waste your £1k than that.