I'm not in Australia, but I can offer some advice
I would not use spray paint. It usually doesn't dry and it doesn't last. Acrylic paint is recommended. You can brush it on or use an airbrush (but they are expensive if you're not going to customize regularly).
Some people prime their ponies but a lot of us don't.
I'm not familiar with that sealer, but generally you want to use a polyurethane sealer. I don't think you can replicate the shine from the original vinyl. Using a matte sealer is best. You can use a shimmer paint after painting the body and the symbols.
Why would you need to put the head in boiling water to rehair? I've rehaired Styling ponies and didn't need to do that. You just want to use a strong needle and go slow. You can also use a larger needle to help widen the hole before you put the hair in.
You would add the hair after painting is done.