could somebody tell me if the personalised/custom plates issued in Victoria are a one off purchase or if the fee, up to $495, is recurring each year.

here in SA Personalised plates are a one off payment of $170 or so and custom is about $190 or so a year.
check out the VIC roads web site it should have all the info there and you should also be able to find out what plates are available.

Its a one off fee, and then just your normal rego each year