Hi Melinda,
This sounds like it will be a fun commission, congratulations!
If this were my commission I'd start with taking a good look at Western paintings containing horses that I admire for their composition. You're right: you are going to have to tackle landscape painting, perspective and composition as well as horse anatomy problems. You are probably also aware that horse owners are very picky about horse anatomy, especially leg structure, so tred carefully there.
I paint lots of animals and I don't hesitate to charge for them, though I charge less than I do for humans. If they are background props I might rethink this attitude - but I would still be really careful about anatomy. Your familiarity with horses will help you out here.
One of my favorite paintings with a horse, by the way, is Caravaggio's "
The Conversion of St. Paul" but I doubt that it's what you had in mind for this commission.
Keep us posted as to how this goes.