A foreign licence without English Language Proficiency of at least Level 4 listed on it cannot be converted until & unless you do the test with an Australian testing officer. Long and boring details are on the CASA website. It isn't a hard test, but there's no way around it. Officially it tests your ability to communicate in English, and to understand "Aviation English" spoken by a less-than-perfect speaker. Unofficially, it's a crock. Holding 3 ICAO licences means nothing to CASA if none of them have at least ELP 4.
You will need the ARN before your doctor submits the medical details to CASA. If you "almost have the medical done", and are "in the process of applying for an ARN", you've got things back to front. You don't mention an ASIC, but I assume you've got that underway.
You will only need to do a flight test if you want an Instrument Rating. You only need to do the CPL(H) Air Law exam because it is a pre-requisite for the ATPL Air Law. You do not need an Instrument Rating for an Australian ATPL(H), and there is no flight test for the issue of an Australian ATPL. Once you have the required exam credits and flight experience, CASA will issue it if you feed them money and paperwork in the right combination.
AFT do excellent materials. They also support you extremely well. Use them. You will need the reference materials. You might not use them all in the LAW exams, but you can't be entirely certain which ones you will need. Well, you won't need the Act or the CASRs for the exam, but one day CASA are going to change that without pre-announcing it, and it would be a complete bugger to find that out when you sat down at the exam. You must have properly bound paper versions of the docs for the exam. You are only allowed extremely limited document tagging. You are not allowed the computer docs, even though that's what any sane working pilot uses.
Agree with the poster who said Chopperline if you need the Instrument Rating. Never studied or worked there, and have no direct association (I know some staff and many students), but they teach up to a quality not down to a price.
Don't mess with CASA, especially CLARC - Just lie back and think of England. This is the organisation that has, in every case I've seen, rejected licence and ASIC applications when the flourish on the end of a signature went even 1 mm outside the signature box on the form. Just to make it more exciting, they won't contact you let you know what the problem is - they'll just snail-mail the whole package back with scribbled notes on it.
Lastly, official advice from one CASA office is not guaranteed to be accepted by a different office. You have been warned ...