I haven't done HHD, but i'm currently doing 3/4 bio now and it is definitely a heavy work load. There is quite a lot to remember, and with concepts you really need to be able to apply them not just memorise. A good amount is also experimental design, and being able to come up with pracs and being able to make good observations to later be analysed in depth. I don't find the subject to difficult (everyone is different), but I think the difficulty comes from the work load and the shear amount of content you have to work through. I still find it quite enjoyable, unit 4 i'm not looking forward to with all the DNA stuff and evolution (is kinda of boring to me) , but unit 3 has been really interesting with it being microbiology.
Seeing as your doing psychology and chemistry, you would already have a few things that would help you in the subject. I also think it is quite doable for going into 3/4 without 1/2. I skipped unit 2 and there is barely anything in that that is useful for 3/4. Unit 1 is was very helpful for unit 3! My teacher went over it again but they also kind of repeat some in unit 3 study design.
Since you trying to get into med, I Highly recommend doing biology. I have spoken to a few people who did med with out bio and they did regret it. It is still very doable, but having biology would make your life easier.
Hope this helps.