Replace Giant MPH disc brakes - mount compatibility

I had a few questions about sizing and IS vs post mounts for replacement disc brakes.

I will be getting hold of a Giant XtC SE bike (approx 5 years old) from my uncle if we are ever allowed to go outside again. The disc brakes need replacing as he has had numerous issues with the Giant MPH Hydraulic Disc Brakes that come as standard. Because this is mainly for my partner to get into riding I didn't want to spend loads so was going to pick up a front and rear set of Shimano Deore BR-M6000 Disc Brakes from Merlin bikes. However, I am unsure of the compatibility of the brake mounts and size of the brakes.

Do I need an IS mount to post mount adaptor or a post mount to IS adaptor? If so, what size? Or how do I work this out? Will I need to by new rotors as well? How do I know what size rotor to buy?

Any help or advice would be much appreciated and if anyone has suggestions about alternative brakes that would be easier to fit then I'm all ears.