our licence is in english, no need IDP.
as for excess, it’s your personal preference as to how much you want to be exposed to. you can look at your own travel insurance to see if you can add on car excess. eg my own yearly plan already includes up to 2k excess. i know AXA single trip you can add on as well.
the car i rented comes with default excess abt 2.7k. i chose not to buy any extra reduction. in this case my exposure is max $700.
look around at the different levels of excess for each car rental companies. usually their base rate doesn’t differ much. but the extra details will matter if touch wood you get into accident.
for tasmania i got full coverage though, including road side assist
and animal damage. really depends on where you intend to bring the car to.