If you're looking into selling at equestrian events, then you are going to need a dye-sub which will be roughly between £500-1000 for a used one depending on age of model and state.
Tbh realistically you are a couple of years from needing your own printer I'm afraid.
You would be much better off building up your own camera kit first, before worrying about printing. If you arent printing onsite, then use a lab.
Your best bet would be to improve your photography and then contact all of the equestrian event photographers in your area to see if they need any second snappers.