The first thing you want to do is educate yourself on other horse rescues/sanctuaries. Online and in person. Find out what you like about them, what your don't and develop a plan for what you'd like to do.
Create a bunch of questions to ask:
How many horses do you take in a year?
How much land do you use?
How much feed to you use in a year?
How much money in donations do you need to offset your costs?
What are your costs?
Many counties/townships have rules about how many horses/per acre. That would give you a good start on the land needed.
Things to keep in mind with rescues/sanctuaries you are likely dealing with elderly/sickly horses. That means money in vet care.
As far as a stallion far, you can do the exact same thing. Look up stallion farms online. Look at all the different breeds. See what kind of requirements they have for stalls, fencing etc. Get experience with/around stallions. After being around them, you may decide that isn't the right direction for you, or you may decide it is.