I am trying to coating metal oxide into steel substrate, for better sticking planning to utilse epoxy based, Could you please suggest suitable curing agent for the coating purpose
Normally, each commercial epoxy resin has their own curing agents (which you can consult from the provider) that allow to obtain a good material. The properties (specially mechanical) of the coating will be determined both by the structure of the epoxy and the curing agent.
Is a very wide question honestly. One of the most popular epoxy resins is DGEBA. The physical state of DGEBA (either solid or liquid or something in between) will depend on the molecular weight that you are buying. The rule normally is: the higher the molecular weight, the more solid it is. To cure it, polyamines or phenolic compounds can be used. Most of them are in a liquid form due to their low molecular weight, but there might be some options which might be solid. It is worth looking in Google basically.
Just bear in mind that using a solid epoxy and a solid curing agent will difficult curing, since you will need them to mix, and mix well. So you probably had to melt both solids to achieve a good mixing to cure and obtain a crosslinked material.