There is loads of good advice out there read as much as you can before
spade hits soil.
Best thing I did on my pond was to put in second level that was easily
converted into a veggie filter.
My only regret is not putting in a bottom drain,
Peter

Oh and personally I'd stick too goldfish, comets and shubumkins, but it
depends how serious you feel like getting, and how much you are prepared
to spend.