Q1 Yes N and G but not form much longer, after April 14 you will need an EASA licence.
Q2 With 500 hours you need to take two exams and UK Flight test plus Flight Radiotelephony Licence, conversion details in
CAP804 Part Q
Q3 How long is a piece of string?
Q4 Basically, do it all over again, so if you want a professional licence you might be better off looking at the Australian requirements first.