SYD, if you trained in Oz, you don't need to spend 2 weeks in NY for an observership. Your clinical skills taught in Oz will suffice for the CSA.
You can take the TOEFL at anytime, the scores are good for 2 years. I would register for and take it now so that the scores are available and you can register for the CSA at your earliest convenience.
In addition, since most residency programs don't care about Step 2 scores and you have some clinical experience, I doubt you will need 4-5 weeks to study for it. Granted if you have been away from some areas (ie, Peds, Ob-Gyn) you will need to brush up on those, but the vast majority of the exam is general medicine. Assuming this is what you've been doing you HO years in, you should be pretty well set for the exam.
In any event, I agree with the others, try not to take the exam any later than August. Scores are now coming sooner but I'm not sure that's true if you take it abroad.