I'd definitely recommend that you check out the Legal study design, and get a feel of what the subject entails. I personally love the subject, so hopefully you will too!!
You can definitely do legal without having done units 1/2. Some of the stuff you cover in 3/4 is just reiterating or expanding on stuff from units 1/2, but there is no assumed knowledge and you start from scratch - so I couldn't imagine it would be an issue.
With regards to resources, I've found the Justice & Outcomes Textbook to be great, as well as the A+ Notes and Legal Checkpoints. And although I haven't purchased it yet, I've heard the CPAP Guide is fantastic - (the author is actually active on the forums regularly [MeganRobyn]).
Probably one of the best pieces of advice I can give is not to try and rote learn the content. A lot of legal entails evaluating elements of Australia's legal and political systems, so memorising information won't be super helpful when it comes to answering the higher-mark questions. So I'd definitely recommend you regularly think and read about the strengths and weaknesses of the things your learning about, and how the different areas of study are interrelated - in addition to being able to apply and understand the information covered.
Best of luck!!