If there are no university-set pre-requisites for the subject, then there's no need to have done it in the HSC for you to continue with it in University. For example, there's no prerequisite for HSC Economics/Business Studies for a Commerce/Economics degree. There's no prerequisite for HSC Legal Studies for Law (take it from me, you pretty much learn the entire 2 years of legal studies in 1.5 weeks at university, and then, there's not really an exam for it).
HSC knowledge -does- help though, but if you want to do a Business degree, you generally should be interested in it in the first place. If not, that's fine as well, as you'll be able to catch up.
And on a second point - if you have no idea what you want to be, that's fine as well. I didn't know which degree I wanted to choose until -after- we got our ATARs. You can still change your UAC choices after ATARs come out - in fact, they're open until like January 5th/6th or something like that.