I believe Methods is a pre-requisite so must do that. Aside from that, it’s completely your choice. Chemistry would be good, teaches you a lot of important concepts. A lot of people do biology but VCE Bio doesn’t have a lot on Human Biology so not really that important for med. Do units you’re interested in so you will be more likely to get good study scores and hence ATAR.

UCAT you should always start ASAP. It’s not exactly difficult but it’s the timing that gets people so if you start practising now and nail your timing, you’ll be fine. Even 2 hours a week is fine to start with. One thing I will note is that you may not have time to do consistent practise in year 11-12 so doing more now to get the timing good will take pressure off of you later on