Hi there,
I sat the HSC in 2014 and came out with a mark of 95 in Legal Studies, so hopefully I can offer you some valuable insight.
How hard is the subject? Entirely depends on the person. If you like the subject, it's going to be easier, and vice-versa. You can make the subject easier for you if you study efficiently. Studying efficiently means focusing on your WEAK areas, not just fine-tuning your strong ones. It also means condensing your notes as much as possible. If you can learn to shorten your notes as much as possible to the point where they are simply 'triggers' for your memory, the subject won't be too hard.
How much memorisation is involved and is it interesting? Most of the essential memorisation is in preparation for the multiple choice, where obscure legal terms often come up. However, the key to memorisation for this section is to start early in the year and let it seep into your long term memory. Get a comprehensive text book with all the legal terms of the first section of the syllabus and make sure you know them. As for the other sections, memorising very, very concise essay plans (literally just stating LCMR) will suffice, no need to memorise whole essays. I personally had a bunch of plans for Human Rights, particularly for the seven marker question, which I remembered in my head. Is it interesting? Again, entirely depends on the individual. Do you like working out the positive and negative components of things like the legal system? Do you like to keep in touch with issues plaguing our society? It's up to you to work it out.
Does Preliminary matter for the HSC? Ha, I can't really answer this to the best of my ability because I was actually ranked almost last for Legal Studies in Preliminary (in Year 12 I was second), due to my consistent lack of attention in class. But hey, I ended up doing okay, right? I'll leave this up to other BOS'ers to decide.
How easy is it to get a Band 6? In 2014, approximately 13% of all Legal Studies candidates achieved a Band 6. For Business Studies, it was 9%. I'm not quite sure whether this means that it's easier to get a Band 6 in Legal or whether it just means that the Legal cohort simply had a greater proportion of high achievers. Either way, you will excel in whatever subject you are interested in, and it appears that if you are excellent in Business Studies right now, you should be able to obtain a Band 6 in that just as easily as Legal Studies.
Hope this was of some help, if you have any more queries, feel free to PM me.