Unless they got for the A320/KC390 mix I don't see much in the way of synergies between the transport fleet (If Im missing something please point it out), Though there may be synergies if they go for 737/P-8 fleet.
In regards to the options out there for the C-130 replacement..
C-130J is the one likely with the quickest introduction time while also having an ally just across the way already operating a dozen of them so while not allowing synergies in NZ's own fleet could allow some savings if worked in with Australia (maybe).
KC390 is still in development, Will be easier engine wise to maintain as it is a commercial engine allowing for part's to be sourced from pretty much any where in the world (One of the reasons the KC-30A's and Wedgetails have been so successful in Iraq has been there ability to source part's quickly). Payload wise while it can carry more then the C-130J it is only in my opinion a margina increase (6 ton give or take) and with the short time frame does present a bit of risk for a small gain.
A400M would be a great aircraft but is still undergoing development so nothing to say it will be able to meet the IOC and FOC time frames. As has been pointed out the 26 aircraft up for sale from Germany and Spain are at the back of the production run, I dont see Airbus increasing production schedules for such a small order so unless Germany or Spain are open to seeling some of there earlier production slots (For Spain with money being tight being able to hold off spending money could be seen as beneficial) I don't see the chances as being good but you never know.
For the C-2 from Japan I am actually more hopeful about, It appears that Japan will have there's coming on line a lot quicker providing comparable range and payload to the A400M at around $40m US less, Benefits of it is theengine is commercial in nature allowing for wider range of sources when needed. If there are no issues in Japan introducing theres then this could be a decent contender.
Personnaly I'd prefer the C-2, gives the best increase in capability, has commercial engines and at a reasonable price.