I just have to add my two cents worth. All of your choices are good options but I feel like the people who take the time to "rescue" the horses (or dogs) and then pre train them and pre screen them so that they are truly ready to be adopted with a much higher chance of success ought to be one of the top choices. I cannot tell you how many times I have rescued an animal, put time into training, vetting, etc only to have people "pass" them up to "save" another. When the animals that I have rescued goes to a new home, it freed me up to rescue another so either way you are part of the "rescue" The question lies in whether you obtain joy from the training, vetting, and other "foster" type roles or whether you simply seek to help rescue while obtaining an animal that can give you the greatest relationship because the animal is ready for that relationship. Due to that mentality I pretty much gave up on rescuing animals and training for others and only rescue when they show up on my farm or for myself. Sad but too many times I had the perfect animal for the individual in personality, training etc only to be told, I really want to rescue one going to slaughter (or to be euthanized, etc). I admire your willingness to rescue and can tell you without a doubt they make the best animal companions under the right circumstances. All of the animals on the farm (save a few chickens) are rescues. My beloved Percheron cross being my favorite to this day. She was 750lbs under weight when I got her and now she is going on 31 years old. She tops at 15.3 and was 1650lbs in her prime. We did everything together until a shoulder injury ended her riding career and I began raising my own children. She continues to grace my barn with the grace and dignity of a retired warhorse having served many years doing a little of everything from trail riding, jousting, war battles, tournaments, camping, parades, riding lessons, jumping, pole-bending, dressage, archery and equine therapy. She was rescued by a friend who found her starved, abcessed foot, untrimmed and recently given birth to a foal.
Good luck with however you help "rescue" and know even though you can't rescue them all, it really really makes a difference to the horse you have.