I can only knowledgably respond to the second part of the question. Basically it's all about the severity of the founder. Many many ponies founder, they are naturally more susceptable than horses. Many, many of them go on to be perfectly usable, saleable creatures. But it can also render them permanently disabled. Only an xray will show the amount of damage done (called "rotation" of the coffin bone) and the vets will then let you know how much work they will be able to handle. A foundered pony will usually have to be kept in a dry lot and their weight watched like a hawk, especially in the summer.
A previous founder will absolutely affect the price of a pony, even if they are sound for the job they are doing. How much it affects the price depends on how badly the rotation, and what the pony's job will be.
Once the acute founder (real name is laminitis) has been stopped and depending on how bad the coffin bone rotation was, the pony may still be able to perform. In my experience, they typically don't return to their prior level. Horses/ ponies that have foundered also usually require careful management feed and shoeing wise to prevent recurrence (feed) and to keep them comfortable. It may be more realistic to lease out a pony that has foundered, although if the pony is a great packer/ baby sitter type people may still be willing to buy. I knew one that had been a good medium pony hunter that foundered and while she was never again a division pony, she had a long career as a short stirrup and walk trot pony. Her value obviously decreased significantly, but she was useful, comfortable and well cared for.
You can Google founder and laminitis and easily find great references that explain it better then I can. Basically its a problem involving inflammation in the hoof and, worst case, rotation of the coffin bone (it can come right out of the bottom of the hoof).
There can be many causes but for most Ponies? Weight and feed play in as well as certain conditions like Cushings, Insulin resistance and obesity that make them more prone to founder. It takes some time for them to recover, most get some pain meds, put on a strict diet and they will be off for quite awhile-anything from 3 to 12 months. There is really nothing you can do to "fix" it except rest, diet and figuring out the underlying cause to possibly treat it. That cause could also be too much work on too hard a ground hence you will see the term "road founder" and that is a little easier to clear up then the underlying physical problems.
Once they founder, it can reoccur. Particularly if there is an underlying condition that is not being aggressively treated.
Far as sale or lease? Well, it it's currently foundered? It can't even walk so that would be a deal buster. If it is disclosed by the seller as a past occurance? It depends on how long ago and, again, what underlying condition might be at work.
You need a vet to take x rays of the hoof and compare them to those taken when it was foundered and see where you are before you even consider it. The meds for those underlying conditions can get pricey in a hurry and the special diet is a pain to supervise if you board out so a blood panel is in order if seller does not disclose any unmderlying conditions...don't trust anybody or assume they even know about the underlying condition.
Anyway, having been around alot of Ponies for quite awhile, I might lease one with a history of founder but only short term (3 to 6 months max) and would not buy one or anything excpet a pasture buddy or lead line for a 3 year old.
When opportunity knocks it's wearing overalls and looks like work.
Laminitis is an inflammation of the tissues that hold the hoof onto the coffin bone.
If this inflammation is severe enough, the laminae will be damaged, and the connection between hoof and bone will weaken, allowing the coffin bone to alter its normal position--this is founder.
So, you can have laminitis without founder, but if you have founder, you have laminitis.
Therapy entails drugs, trimming and shoeing appropriately, and sometimes surgery.
A horse which has foundered may make a significant improvement, but rarely do they recover completely.
They are also at higher risk of a repeat episode.
I would be reluctant to advise anyone to purchase an animal with a history of founder.
Here's a nice intro to the basic structure of the hoof. Here's a web-book that gives a lot of information regarding laminitis and founder.
"It's like a Russian nesting doll of train wrecks."--CaitlinandTheBay
My horse got laminitis this past September. We still don't know what exactly caused it, maybe an infection from a small cut, we don't really know. He rotated behind, but not in front which is somewhat unusual. Once he stabilized he started walking much better. The vet said it would take 9-12 months for him to grow a new hoof and then he would be back to showing and jumping (he was a high a/o 1.50m jumper). In november he had a relapse after the farriers did some work on his feet. We had to put him down and the bones had by that point come all the way through the sole of his hooves.
I do however know several ponies that have foundered and now are completely fine. The vet made it sound as though if nothing got worse, as it did, we would have to wait for him to grow a new hoof and then he would be fine again for showing, jumping, and resale. In the mean time he was on stall rest, got special meds, and we had farriers who specialized in laminitis working on his feet to help correct the angles so as not to stress the ligaments in his legs.
I'm no expert, but that is my terrible experience with laminitis and OP if you have or know a pony that has foundered I wish you and the pony the very best of luck.
Many horses now a days who have foundered can go back to their same level of work once recovered. The biggest thing to take into account is what was the cause of the horse to founder in the first place. Infection, retained placenta, mechanical, cushings, IR, etc. Once identified managing that issue greatly reduces the reoccurrence of founder.
There has been extensive research done by Esco Buff on how to trim and shoe a horse who has foundered and his success rate has been very high. Hopefully, with more time and research this disease can be prevented or cured entirely.