Not a problem Matt, that is good news the water level in the overflow stays at the same level and it is not using and or leaking any water, or pushing any out of the overflow bottle.
As it has gotten hot before and no doubt will do it again, it is good that you have ordered a thermostat (standard is ideal I think, but I have no experience racing with a nismo low temp one) to atleast eliminate that straight away. For the sake of a few bucks I'd still get new coolant fitted making sure it's bled correctly, and pressure test the system. Also ask the mechanic to check that the radiator cap is actually holding pressure. It's an easy check you could do youself if you wanted though like I said just by squeezing the radiator top hose once your car is up to operating temp. It should be quite firm, not soft like if you squeezed it first thing I the morning when the car is cool.
With that amount of kms I would be surprised I'd the water pump blades would be corroded especially if it's always had good quality coolant fitted but stranger things probaby have happened and may be worth investigating as a last resort if everything else fails as time can be just as much a factor as km's sometimes,
I have zero experience with the n1 water pump (moving less fluid at low speeds) but I don't think even and n1 should be making your temps get that high.
Good luck with it