We do a bunch of this sort of work for people.
M25 valves are available but 99% are 3/4'. all valves require burst disks and this is enforced - it is illegal to fill a cylinder with out one.
Regarding colors there is no legal requirement so anything goes. Correct labeling of gases is generally required - Nitrox, o2, trimix etc.
Generally there are only a limited size range of cylinders available in NZ - faber or Euro 8, 10,12,15 L all 232 bar. Ali 40, 80, 95 CF - Catalina and Luxfer.
If you have say twin 12L Fabers then leave them behind as new cylinders and valves are around $400ea
If you have 300 bar Cylinders or some stages that you like it is best to email me direct - firstname.lastname@example.org
and we can check out the options and costs involved wit bringing them out.