These are all pure distilled and have no inhibitors to stop growth/gunk and corrosion. Get the mayhems x1 clear as that has all the inhibitors needed to stop corrosion and growth/gunk. Just make sure you get as much air out as you can and fully clean the old unit so you don't make thing potentially worse. Also 1 litre is plenty. The 240mm have 100-150ml and 360mm have 150-200ml.
This the one you are referring to so no messing about once I have flushed it i just fill all the way with that nothing else?