98 is a really good ATAR! Yes, you can repeat your HSC up to 5 years I think, and no, repeating won't ruin your chances, but is it really worth it?
Do you have to do medicine at UNSW?There are other uni's that offer it: Bond, WSU etc. Any other interests besides medicine?
Your chances getting 99+ will be the same as a normal high school student, you won't be disadvantaged or advantaged.