Not sure what the rules are in New Zealand but a couple of things that don't seem to be mentioned here.
1. You won't be able to buy this property using a mortgage as its a development investment. If you both have the funds to buy then fair enough but i would have thought NZ Banks would see this as a business loan not a mortgage.
2. Not sure whether NZ has an equivalent of Building Control, but from what you have said it sounds like an earthquake damaged property requiring significant structure work (Piling). If that's the case in the UK building control would not let anyone 'live' in it until suitable for occupation, which is likely when the building in structurally sound, safe and has statutory services.
3. If you can make it work, make sure you employ a structural engineer to design or at least consult on the repairs. You may also need a Party Wall Surveyor if its not a detached property as if your work causes damage to neighboring property you are liable and make sure you have the appropriate insurances in place.