is blender where you are going to do the final rendering? it totally is fine if it is but as you said "a game i am making" i suggest you put the mesh into the engine and work with the shaders in there. unless you plan is to replicate whatever you do in blender in your target engine, which is totally fine.
honestly dont go into retopology before you have seen what it looks like. the engine will handle the geo take it in as early and often as you can, as you can not predict the look as easily just inside zbrush.
very good start for the first sculpt ever! my main question would be, are you going to render it using cel shading? if so get it in engine asap. doesnt matter if its too highpoly. you need to see how the shader reacts to his surfaces as early as possible