As several others have said, be very very careful. I had a horse come back from EPM many years ago, before Marquis, etc was available. I was very lucky, and had parents who were willing to spend a hell of a lot of money and time on treating him, and he returned to third level dressage. I did jump him occasionally post-recovery, but more for fun as he was primarily a dressage horse. He was diagnosed at I think 20, and the EPM was not a problem again until his death at 34. However - I was neurotic about preventing stress in his life, and never letting him so much as see a steroid, even when he developed lung problems that would have benefited from steroid treatment. After scheduling euthanasia (due primarily to breathing problems), we decided to treat with steroids for his last few days to make him as comfortable as possible, knowing that it would take a few days to trigger a relapse (which, according to most vets, after that many years it should not have). It did, and the morning that he was euthanized he had completely relapsed and had full-blown neurological symptoms.

So, just know that: your vet will likely want to avoid corticosteroids always, which may become an issue if joint injections are needed to maintain soundness; the horse could have been misdiagnosed and have some other neurological issue, EPM was for a while the disease of the week and blamed for a lot of mystery lamenesses; the horse could be fine now because it is pastured, but relapse under the stress of work; if the horse relapses, you should be prepared to spend $1000+ if you elect to treat; if you want to sell the horse, potential buyers are likely to have all of the same concerns.

For a long time, I was very involved in helping other people whose horses were diagnosed, and a good friend's horse is currently dealing with it. There are some horses that recover and never have another problem, some that recover but are never quite the same and are often retired or need less demanding jobs, and some that never recover. And it's very difficult to predict which category any horse will fall into. I wouldn't necessarily not buy a horse with a history of EPM, but I would definitely give it a lot of consideration and want a LOT of detail from the owner, a very thorough vetting, etc. There are lots of other horses out there without issues, and EPM is a lot of baggage to take on. However, from a purely rational perspective, buying a horse at all is generally not the greatest financial decision , and you could buy a perfect horse with no issues and a perfectly clean vetting, and have it step wrong and break a leg getting off the trailer. There are no guarantees with any of them, unfortunately.