I've spent many hours scraping and welding the floorpan and sills of my Bermuda Blue 1969 mog and found the panels to be invariably painted matt black, with red oxide primer underneath. All the colour pictures of restored cars I've got to hand show the inner wheel arches coloured black or dark grey, rather than body colour. A few black&white 1950s publicity photos however show the wheelarch to be painted body colour without underseal. The choice is yours, but I would underseal it all if the car is for all-weather use.
The first thing to do is coat the (bare metal) underside with a thick layer of anti-rust primer, preferably one with copious quantities of zinc. As others will not see the underside, I would be very tempted to brush paint it on, working it into all the nooks and crannies. Then it needs an overcoat of very hard wearing, chip resistant paint. Hammerite do these kind of products in the U.K - there must be similar products in New Zealand.
Finally a nice thick covering of underseal will protect the underside for a few years (until it dries and starts cracking).