Christ
If you do a subject e.g. Health and Human Development , it is really easy to do well in (If you are good at writing and explaining) - scaling don't matter if you are comfortable at a subject. I believe that the higher you get for a study score, the less in scales down.
I don't score less than a 95% on every SAC.
Regarding Health- it is achievable to get a 45+ study score with not too much time and effort ( in comparison to a subject like 3/4 Chemistry, that tests you in a multitude of ways).
I believe Biology is not a prerequisite for medicine - but it is good to pick if you are interested in that subject.
The best advice is to do subjects you like - not because they scale up or down, but ask yourself if you are willing to keep up with the amount of workload, able to bounce back after setbacks, constant motivation and proper time management.