My cistern looks so bad/good that it should probably just be discarded/doesn't need to be checked?

You shouldn't judge a cistern on the outside, it can be flawless inside even if the paint has flaked on the outside. Unfortunately, it can also be the case that a cistern that looks good on the outside can have very deep damage on the inside. It is usually inside the cistern that it rusts the most and therefore it is there that the most important assessment is made.