If the view of the NZDF is to upgrade the aircraft, why bother employing a private contracting firm to assess whether new aircraft should be bought, or old aircraft upgraded?
Being the cynical and suspicious person I am, I wonder if perhaps this independent contracting agency could have been L-3 Spar by any chance?
In any case you are still missing the point. Your C-130H fleet is becoming increasingly obsolete due to the size of the cargo hold, the size of it's fuel tanks, the efficiency of it's engines and general design.
The upgrade addresses none of these issues and what you will get is an airlift capability identical to now, but slightly more reliable.
With a quick glance at your other defence acquisitions, the platforms being acquired to provide mobility, protection and firepower for your land forces are not designed for air transportability in a C-130 and therefore you are spending a large amount of money on a project that delivers very little capability enhancement.
It's akin to Defmin Nelson's off cited example of upgrading an EH Holden with modern Commodore components. At the end of the day, it's still an EH...
On top of which your sums are wrong. At the current US-NZ exchange rate according to universal currency converter, US$304m equates to NZ$402m and this is the price Norway is paying for it's fleet for 4x C-130J-30's. The capability of that fifth C-130 must be KILLER at more than $600m for a single trash hauler. I don't think a C-17 even costs that much...
Try dividing NZ$402m by 40 years and see what the yearly rate works out to. Significantly less than $5m per year I think you will find...
I'm not advocating this for NZ mind you, the C-130J-30 doesn't improve on the C-130H's ability to lift heavy loads, it just allows the C-130J-30 to lift the maximum weight it was intended to lift , nor is it's payload / range specifications in the league of the A400m, let alone the C-17, however it is a large improvement over the C-130H.
I would not at all be surprised to see that 4x C-130J-30's offer greater sortie rates than your upgraded C-130H's will and when combined with range and cruising speed increases, I'm sure even you can see that it's the better airlifter, despite what your Defmin might have stated about it...