First, a 501(c)(3) is a good thing to look into if you're looking to be non-profit and rely on donations and your own funding. Most reputable rescues have a 501. You can talk to owners of other reputable rescues to ask how they got started and if they have any advice for you - Section 501(c)(3) Organizations
If you plan on starting this a few years in the future, I would plan for facilities. Are the horses going to be kept on one property belonging solely to the organization? Are you going to allow horses to be fostered on outside properties (in this event you would have to be careful for people trying to steal horses if there is no valid legal paperwork documenting that the horses are currently the property of your organization and not the farm they are staying on)? Will you be able to afford full care for multiple horses if you receive few to no 'sponsorship' donations, especially horses that may have special dietary or other special needs? What about possible emergency vet visits if something happens? Will you have volunteer staff? Will the horses (that are able to be worked with) receive training/exercise/riding while in the care of your organization?
Definitely not trying to be a downer. I think these are things that are important and must be thoroughly thought about.