HHD is probably one of the most ideal subjects for jumping straight into 3/4.
I did 1/2, and obviously, anything from 1/2 that you need in 3/4 is covered again extensively. There isn't any assumed knowledge, or rushing through something because 'you should know it from last year'.
Basically, the advantages of doing 1/2 are that first of all, you get a feel for the subject and how to answer questions and analyse graphs, which is a pretty general skill. Also, the tone or focus of the subject kind of changes from 1/2 to 3/4 so even that advantage is limited.
The other advantage is that you already have an idea of some of the things that are covered in both years, and doing them in 3/4 serves as a kind of revision. But again, they're covered both years, so meh. Things like:
- Definitions right at the start of unit 3 for health, the dimensions of health, health status, measures of health status (life expectancy, mortality, morbidity, burden of disease, etc.)
- Nutrition (Just the nutrients, what they do, which foods we can find them in)
- Medicare and Private Health Insurance
To sum up, yeah go for it.