NZ is an excellent place to find Fiat 500's or the 'Bambina' as they were badged over there (for 500F models)
They can vary enormously in condition depending on what part of the country they have lived in.
I imported 3 over from NZ last year, 2 excellent rust free examples, one perfect, one needing a repaint and one very solid restoration project.
If they have the original registration papers it can tell a lot about the probable condition- Generally Wellington cars are most likely to be rustiest due to lack of garaging in the hilly terrain and the sea air blasting in as can West Coast cars from the South Island.
Auckland cars can deteriorate more due to humidity.
Christchurch and Central South Island cars are often in excellent condition - my 1972 500F was garaged from new in Christchurch and has no rust or repairs anywhere.
The disadvantages are that the stocks of good cars have been severely depleted especially during the '90s when large numbers were sent to Japan. I looked over at least 10 prior to selecting the ones I imported.
The freight costs out of the South Island are also astronomical due to the lack of roll on - roll off facilities in Christchurch which by the time you have cleared customs, paid GST and import approval it is a marginal economy compared with buying one locally.
As usual buyer beware- just because it is glossy it isn't necessarily solid. Having said that - if possible look for a car with a Warrant of Fitness as there are compulsory roadworthiness checks every 6 months in NZ which are a reasonable indicator of condition.
Good luck!