More than once I have thought a section strictly for Seniors would be a good idea.
18 and up might be the starting point, since "things" seem to start happening when a horse hits their late teens - if it's going to happen.
I took my first two Keepers the ages of 27 and 29 (the 29 year old was born on my parents farm when I was 13). That was in the early 80's and, except for a few stitches, they both were extremely healthy until their last 2 - 3 years.
My current Crew of Keepers are 17, 18, 25, 26-1/2. The three Walking Horses are frought with issues. The Arab is 26-1/2 and has some issues, as I rescued him 19+ years ago; at this point he has become the healthiest horse in the barn. I feel like my barn has morphed into a Skilled Nursing Facility for Horses. On to the OP's very important questions:
Anytime a person rescues, you have to take that horse in with the knowledge that you don't have a lot of knowledge regarding the horse
1) Putting weight back on a horse is the easy part.
2) Giving the hooves proper care might be the next easiest part -- unless the horse has foundered at some point in time, then whomever does the hoof work needs to be well versed in how to managed the hooves.
3) Many of these rescues end up with ulcers - acidic digestive juices keep working whether there's any food to process or not. Stress of abuse/neglect can induce ulcers. The new owner has to be on the watch for the slightest of symptoms. Vets can't determine ulcers with 100% accuracy unless the horse is scoped but, there's enough other indicators (even to the owner) that ulcers can pretty much be determined.
There are two types of ulcers: Gastric stomach ulcers are the most common and VERY common - even in well-cared for horses. Hind gut ulcers are not so common and more expensive to deal with.
4) Not all feeds are created equal -- STEP AWAAAAY FROM THE SWEET FEED!!" If I could lobby to get rid of anything in the horse nutrition world it would be sweet feed. Have a little grain with 50 pounds of sugar. It's like feeding a ten year old boy HI-C and Hershey Bars every day until he's old enough to get his driver's license - provided he doesn't develope diabetes by that time.
4.1) Horses DO get Type II Diabetes except it's called insulin resistance. That is something that could easily be lurking in a rescue horse as it's very common these days in horses that get the best of care.
4.2) Best to feed a Ration Balancer that has all the vitamins/minerals a horse needs. Even though most RB's already have a pre-probiotic in them, I would buy one and it the recommended dosage to the horse's feed pan anyway.
All senior horses will benefit from a pre-probiotic and rescues even moreso
4.3) Feed quality grass hay - stay away from excessively stemmy hay, if at all possible. Senior horses do NOT have the same chewing ability as they did when they were younger, regardless of whether the vet says those teeth "are good to go" or "should be fine". The fluffier the hay the better, you can even wet a down a bit.
Senior horses often benefit from some alfalfa. You could either buy Standlees alfalfa pellets at Tractor Supply or Standlees timothy/alfalfa cubes at Tractor Supply. Cubes need soaked down to much so the horse doesn't choke.
Forage, forage, forage - horses were designed to eat forage not to have their feed pans filled with horse feed - we humans thought up that feed business.
But in this day and age, horses do need vit/min supplements which is why a Ration Balancer is a good idea.
If the horse has trouble holding weight calcium fortified equine rice bran can be added at the recommended amounts on the bag. Tractor Supply carries Manna Pro's Max-E-Glo.
There's also beet pulp pellets that do not require soaking like regular beet pulp does.
5. Watch for stiffness. Even the healthiest of horses develop arthritis. That would be something for a new horse owner to address with the vet. We all have suggestions but consulting with the vet FIRST to get an accurate diagnosis is important.
6. Lastly comes inevitable gut-wrenching knowledge of knowing that, someday, the horse(s) has to be sent on to its ancestors.
What plans will you make for that? Do you have room on your property to bury them - are you zoned in such a way that you're permitted to bury livestock on your property? If not, who would take the horse away once the vet lays it to rest?
I hope I didn't scare you away from your desire to give one or two seniors their Forever Home but these are all "what if" issues you and your checkbook have to be prepared to deal with.
The only absolute you have to think ahead on is #6.
Please keep asking question and good luck on your venture