Well, to give you some perspective as someone who's flown the Caribou, E,H and J Herc (before proceeding to "greener" pastures!
), and the CN235 with the IPTN chief test pilot when it came out here some years ago trying to sell itself as a Caribou replacement, I really just don't think there is any aeroplane at all which can truly replace the Caribou.
At least not in exactly the same role with the same capabilities. They just don't make fixed wing aircraft of that size which can do what the 'Bou does anymore. If they do get the C27J, I'm not saying it will be a bad aeroplane, but it will clearly be a compromise in payload versus short/rough/soft field capability, no matter what the manufacturer says.
As for the H. Not long of this world I'm afraid. A "few" years if they're lucky. There are all sorts of retirement dates being bandied around, most of which are hugely optimistic IMHO, and will cost enormous amounts of money to achieve. It has been a great workhorse, but the old girl has had a demanding role and has become pretty battered and bruised. It was only ever destined to be replaced by the C130J (despite the vehement protestations of certain Navs and Flight Engineers!) as the Herc is a fundamentally good design for what it's supposed to do.
I think it'll be a good transport force if it eventuates that way. The C-17 is a good aeroplane, even if it was only with the benefit of hindsight that the RAAF ended up with it. It was purchased in a real hurry because the Government finally woke up to the fact (after several years) that it couldn't keep sending troops and equipment on big deployments to the four corners of the earth, without a true long-range transport and having to rely on contracted Antonovs, a couple of rusty clapped-out 707s
, etc, etc. I just hope they continue to fund it logistically.