If your horse is one that progresses slowly and does well on Prascend, he may be quite easy to care for if your parents are willing/realize the importance of/ and give him his pill each day.
I put my mare's pill in a carrot, but when I go on vacation I buy pill pockets so I can put a month's worth of pills into treats, and that makes it easy for whoever is feeding her.
I've heard some people use half a fig newton to put the pills in, and horses tend to like them.
My mare is 28, and I first noticed signs of Cushing's when she was in her late teens. At about age 21, we started her on a half Prascend pill, which she took for several years. Then we went up to a whole pill, and she's been on that dose for several years. She does grow a winter coat that is longer than normal, and requires some help to shed it out by late spring. I use grooming blocks and rubber grooming mitts to help get the hair off.
My mare has not developed insulin resistance, and has never had laminitis. Before starting on Prascend, she had pneumonia once, a neck abscess once, and would frequently get scratches. She was beginning to lose some muscling on her topline and had a pot belly, was not able to sweat when it got hot, drank a lot of water and urinated excessively. She is still easy to keep weight on and does not need hard feed, only hay and pasture.
When people talk about Cushing's, there are many cases that progress very slowly and the horses only have minor issues once on Prascend. I had another mare who foundered a couple of years ago, tested negative for IR and Cushing's, foundered a year later, tested positive for IR and Cushing's, and I had to put her down a month later. This seems to be another variety of Cushing's (possibly a tumor rather than just pituitary malfunction, as
said), and is difficult or impossible to manage.
Also, horses that have insulin resistance along with Cushing's seem to have more issues and often develop laminitis, which can make a big difference.
Unless you knew a person who was a great horse owner, and that you trusted to take over ownership of your horse, I would hesitate to sell a horse with Cushing's. Even with the minor cases, the pill is a monthly expense that is necessary for the horse. From what I've seen of a couple of untreated Cushing's horses in our area (super long, patchy, shaggy coats, gimping around with chronic pain from laminitis, muscle wasting), a horse is probably better off being put down than dealing with those types of chronic issues.