Chemistry is a piece of piss. Physics I found even easier. Chem in prelim is ridiculously boring, but easy. Physics in Prelim is a lot of maths, not a high level of amths, if you have strong algebra and can solve moderate equations than you will nail physics.
Chem is more content based, problem solving (at least in prelim) is basically only balancing chemical equations. It's the sort of subject where by you can read the excel book two days before your yearly and still do damned well.
This is jus my experience of Chem/Physics, some people find either one or both really hard, we had a fair people few drop from them at our school.
As to what you should choose for Yr 11. It entirely depends on you.
In prelim I did
Ext Eng
Ext Math
Modern
Ancient
Chem
Physics (Dropped Chem for physics 5 weeks before the end of prelim)
Legal
SoR II
So I'm probably in a good position to advise on differrent subjects for prelim.
You have to do English and do at least Adv because of scaling
I found Legal really interesting and love it
I'd do Modern over Ancient if you want to do a history
Do both chem and Physics, at least to start with
If you want to do Medicine do at least 2u maths, i'd suggest 3 U even if you know you're going to drop it later, it'll help with physics and 2U
Then whatever you want, I'd never do Art, Drama or Music, but if you like them do it. Just keep in mind that some subjects have a capped contribution so even if you get, for example, 100 HSC score in drama it'll only contribute about 95 TES.
Ext Eng is a piece of shit, kind of interesting, will sort of help with Adv but I wouldn't do it for HSC. it takes English to the point where it just becomes meaningless
SoR is an easy content based subject, if you can write an essay you'll kill it. I wouldn't bother though unless you're school makes you.
I would suggest doing a heap of units, most people do 12, but seeing as you're not sure what you want I'd advise do more and just drop what you don't want to keep. Its prelim, you're not going to have to do a hell of a lot of work, 16 units is easy to keep up with. (Different story come HSC!)
Sciences are more gauranteed marks than humanities, if you put in the work you'll get the marks. Humanities are a bit more iffy as they're way more subjective.