Paint’s not the worst thing out there, but be aware that it will chip and flake over time. Unless you don’t care at all how the gun turns out, you’ll need to do proper prep for paint to hold well, so that’s a full clean, scrub, buff or bead blast, then degrease before painting. It’s time intensive, but you can at least do it at home if you’re willing. You can do less, you’ll just get poorer results… comes back to what you want at the end of the process.
A step further is cerakote. I’ve never gone this route, but it seems to be the leader in paint-on gun finishes. There is an NZ supplier.
For the level of effort in prepping a gun for painting, look around for any companies that do phosphating/parkerizing. This is basically bonding zinc or manganese to your base metal to resist corrosion. It holds oil well on its own, or you can use the phosphated base layer and bake or bond a paint over layer onto it for an extra tough finish. There’s a bit more to it, but that’s the basics.
Hot bluing is another option (like phosphating you’re probably going to have to contract it out), but if you want a working gun I wouldn’t bother. For hot bluing to look good it takes a lot of surface prep, and for a working gun it’s just going to get scratched/rust again. General rule for hot bluing is that the metal will look exactly like it does going into the blue, it’ll just change color, so any rub marks, scratches, pits etc, will still show after bluing. In this case, I’d go towards phosphating with an enamel/cerakote - it looks fine and is tougher in my opinion.
That is provided you don’t mind the time, effort and money… a bottle of meths, a chunk of brass wool, and a spray can of black enamel paint will also give you a sort of rust resistant surface finish for far less effort… it’ll just be ugly and chip off over time… comes down to what you want it to look like at the end.
Avoid cold-bluing compounds, they do about as much as a vivid marker. Cold bluing is basically just painting a layer of selenium dioxide onto the metal, which rubs off as easily as it goes on, and in my experience also rusts as easily (or moreso) than bare metal. I understand it has its uses, I’ve just never found any of them worth the effort involved compared to other surface finishes.