I worked with a herd of 8+ for 4 years. They are super adorable.
Their temperaments vary as much as other horses, but they tend to be strong. If they are bold, they are really bold. If they are shy and skittish they are really shy and skittish. If they are really reliable and even tempered, then they are priceless.
Several did have summer eczema issues. Which can be a real nightmare. Just about impossible to prevent. And even with very diligent treatment, there is only so much improvement you can make.
One of them did have heaves, which was up and down. Sometimes able to be in work, sometimes out of work for months.
As far as gaited horses and humans with arthritis, I would recommend your husband try out a few gaited horses first. Gaited means the bouncy trot is mitigated, but gravity still applies. The rider still has a body they have to organize on the back of a moving horse. And a running walk/flat walk/tolt, etc does not make gravity disappear. A wide backed Icelandic may be a lot more problematic than a slab sided TWH, Missouri FT, etc.
With my herd I clipped them 5-6 times/year. Their coats are substantial. So if you are in a cold climate, that could be great.
The Icelandic gaits have been identified in the DNA, so if you get something with papers, you can get a reliable indication of how strong the tolt should be. I would recommend staying away from anything that does not have the genetics for a reliable tolt. They often demonstrate it in the field as their preferred gait. So a seller that claims to have an Icy that ‘does not tolt but is in training’ would be a NO. They may do it eventually. But it will always be more difficult for them and they will prefer not to.
It is hard to tell what other people like, or what will work for them. There are breeders in the USA, so I would strongly suggest a trip to a breeder to them out. Same with the other gaited breeds.
Good luck. I really enjoyed my time with the Icies. They are super adorable little beasties.