Bend a lot
The C130 operating costs would depend on who owns and operates them them. Have a look at the Fire Aviation site for the story of the ex US Govt Hercs being converted for the US Forestry Service and the fiasco that its become. I'm sure that our mate 4 holer would be able to use this as an example of how its done in the US.
Overall final costs per hour of operations are the result of many things including purchase price of the aircraft and its ongoing maintenance. If the Hercs were so expensive I doubt if Coulsens would be running them. If the Australian Govt had kept the C130Hs, then the purchase price would have been 0 thus making the concept viable. The same for the Orions. At least 3 Canadian operators and one US operator have L188s/Orions. Having worked as an LAME on both L188s and C130s, I don't see that the costs of a C130 would be much different to L188s.
The Q400s may be suitable but the purchase cost must be much greater than the cost of a B737 300.
The Bae 146 is cheap to purchase and the fire engineering seems to now be pretty good, with a wealth of type experience in Australia
As I said earlier, first thing to look at is the model. Federal or State. Common sense would indicate Federal, but the chances of common sense or even reality in that arena is a big, big ask.
Federal would enable larger aircraft in a pool. Possibly the US model where the US Govt provided the airframes for conversion and then operated by contractors. That was a good system until the CIA got involved. Hopefully that type of thing is not a problem here.
Having sat at a roadblock and watched small aircraft try to contain a fire and then watched the DC10 knock it down in one pass, I must admit I'm a convert to large aircraft in the right circumstances.
Wunwing