You are right, it is one of those things that is hard to get a definate answer on.
I currently have a 5 yr old Arab gelding with stringhalt in his right leg. He as most arabs do, has very straight legs, which also puts more stress in general on them. Whether it is related or not he has bog spavins in both hocks.
I have not found any relation in his workload directly related to his episodes, although he would be considered to be in light work 3x week about 45-1 hour each time. In a ring, so no hills. As far as the dandelions, that is funny. The owners who have a small 15-20 horses all nation quality Arabians feed dandelions as a treat. They actually pick them, cut the root stem off and feed, maybe daily. That has not effected him adversely either. Research does say- that removing the horse from pasture may help.
The only thing that I have had generally consistant help with are the Norfield magnetic hock boots. When first used, just because of it being different he would really elevate his leg, which would trigger a spasm. After he was used to wearing them, he wouldn't do that "knee jerk" reaction. We used mostly for maintnance, but if not used for awhile and he starts with spasm's, used for 3 days consecutively, he wouldn't have another for apprx. 2mths or longer.

On a personal note-you say there is a 3 yr old that you really like. There are enough good horses without medical issues, that if you are thinking about buying her, I just wouldn't.