vet visits every 6 months for vaccinations
Farrier every 6-8 weeks
Dentist every 6-12 months
Worming every 30-60 days. Youll want to do a rotation plan so you'll be changing the active ingredient each time
You'll want to ride 4-6ish times a week
Besides board, which can be from 100ish-1300 a month expect to spend extra money on new equitment for the horse, or supplments or medical stuff
Many horses require supplments, like joint or digestive health, so make sure you ask the person who owns the horse your interested in if he's on something already.
You should try to know basic health carefor a horse. Like how to deal with cuts and wounds, how to tell if a horse is lame, etc.
Get a horse suitable for your riding level, not just whatever one is the "prettiest".
Youll need to be able to handle it on even its worst days.
I usually recomend first time horse owns to look for horses between 12-17 years of age. Young horses require more experience for training so slightly older ones are easier.
Owning a horse is alot of work, but alot of fun. Good luck!