Hammerite, single pack paint, dries by evaporation, sticks to a keyed surface, dries hard. Therefore it has a tendancy to chip easily.
Acid etch primer, two pack paint, dries by a chemical reaction, burns itself into the base metal, does dry hard (by comparisson), though it is tough. 2 pack top coats are very tough, but they are flexible, which is why i did not describe the finish as hard, as i did for Hammerite. The choice is yours....
I've never heard of a bodyshell being powdercoated, anyone else? The finish isn't as impressive as 2 pack paint, so i don't think your body panels would look all that?
The body shop/paint shop should be able to mix your colour pretty much spot on. I'm assuming your car is it's original colour, which is why i said quote the paint code, so they can do as good a job as possible.