I went for a class-2 medical because I hadn't had a decent medical checkup for many years. The mistake I made was disclosing 70 years of medical history that they used to put me through the wringer. After months of tests and analysis they were calling for more so I spoke to my GP, he said "Don't worry there's nothing wrong with you, I'll write them a letter". That did it, I immediately passed but it only lasted two years so next time I just did the class-5 declaration.
Your flight instructor will direct you to the correct quack for your CASA medical. Mine flight instructor told me, after the fact, that you just tick all the right boxes and you'll pass. I thought that might be ok until you attempted and insurance claim and some relevant medical history was disclosed then you'd be stuffed.