Bend a lot.
I know they are 4 engines. I said that I was an LAME on those types at one stage.
Getting a military type on the Australian register is only as difficult as the Feds /CASA make it. If they want them there it will be done.
Given the number of 4 engine types in Canada and the US there must be a reason for it. Free airframes would contribute I suggest. I'm sure outfits like Buffalo, Conair and Airspray don't do it at a loss?
Having said that the B737/Bae option seems a good one. I still think that a standard type is the way to go for the large fire bombing fleet and I still think it should be Federal.
The French Securite Civile is an interesting model to work from.
Wunwing