In my opinion, the best time to do a Bachelor of Medical Science is never. You end up accruing another year of HECS debt and spend another year in university while you don't have an income (and Youth Allowance is not an income). If you're intent on doing research, why not just wait until after you've graduated so you can get paid to do research?
Oh noes, HECS debt. Wait, I'm going to be well paid out of uni.
The point about financing yourself is valid, but I mean, it's one more year of uni, not three or something. Surely there are ways to get supplementary income on top of Centrelink (shock horror, part-time job?).
I don't think the main point of the BMedSc(Hons) is to do research, it's to learn how to do research (including annoying but necessary stuff like grant writing, ethics etc. etc.). If you've spent a year in uni learning how to do this, you're going to be at a much better advantage when you do research as a clinician - from all my observations as a child of two researchers, it's not something that you just pick up. And if you're good at research and getting published, then you're going to have a better chance of getting into your preferred postgraduate training program in these increasingly competitive times.
I still haven't decided whether I want to do it - mainly because I'm not sure how good I'd be at it, and also because you get a year behind your original cohort - which kinda sucks, although that's not such a big problem if you say do it after fourth or fifth year.