First time replying on Atarnotes!!! XD
Just assuring you again, I'm doing HHD now (going to have my trial exam today...) and I didn't do it last year--I didn't even do Health before VCE--I'm not too sure exactly what that one's about, but everyone in my school did it unless they were in the class that chose to do Latin, which is me

Having abandoned all science subjects as soon as I had the choice (no I didn't actually fail any of them), I mostly chose HHD just because I had to choose a subject and it seemed easier than my other options, and would also be a change of style to the rest of my subjects.
I spent a lot of Units 1&2 complaining about the whole subject, but Units 3&4 are actually quite interesting, and you start to see how things from 1&2 do actually connect.
I've been doing pretty well in SACs etc, and my general comment would be that there's nothing particularly difficult about HHD, but you just need to put in the time and study it (more so than subjects where you can sort of depend on performance on the actual day, if that makes any sense?)
You'll be fine taking up Units 3&4, plus you've got atarnotes XD
I wish I found this website earlier in the year! All the best!