I suppose as with everything it's a matter of choice - I personally did Medentry and I guess it was useful but I'm not going around beating its drum and saying I would have been a lost cause without it. I think more than anything with me it allowed me to get into the mindset of doing the UMAT and the type of questions involved - there are many other ways you can do this e.g. mensa puzzle books, etc but I'm a pretty lazy guy so I probably wouldn't have been motivated enough to do anything if I hadn't gone to the prep course.
Prep is by no means necessary - I know quite a number of people who got much higher UMAT scores than I but who didn't attend any prep courses.
As for interviews, number one; UMelb doesn't offer undergrad anymore. UNSW is a bit of a weird interview (the kind of interview you either really like or think is completely pointless and a waste of time) - very much a psychological kind of assessment of your personality.
From my point of view, and others may disagree, the interview (generally speaking) is very much a measure of how you have learnt from your life experiences and your awareness of the world around you. These include appreciating types of skills/qualities you have acquired from being in certain situations e.g. teamwork, leadership, communication, mediation as well as acknowledging and understanding yourself, your motivations, weaknesses, etc as well as a sense of your 'place in the world' - they want to know that you know how to conduct yourself appropriately in the real world (and have an appropriate mentality and temperament to be a health professional) and not that you're some narcissistic maniac. Also that you know what you're getting yourself into.
