Yours fans cycle ok so this shows the temperature to be ok, as said the gauges aren't brilliant but they do give a rough indication (It may just be a iffy sender).

Any water cooled engine left idling long enough will overheat due to insufficient airflow, this is the reason for the fans, the gauge should not however creep up with the fans running it should actually decrease.

Good idea changing fluids always good to do on a car you don't know history of next time it may be worth 'Reverse Flushing' the system as well.

Martin