The barn owner of my stable has zero to do with the horses on the property (except for a few mares he has at the back of the property). He maintains the property, cuts hay, interacts with the boarders on occasion, and fulfills small requests that boarders may make (he built us a mounting block, fixes gates, waters the arena sometimes, etc).
Boarders pay a fee to the barn owner for boarding/hay, and a fee to the barn manager for horse care, feed, etc. She does all of the work with the horses, and with the exception of his few mares he never touches them. If a horse was in obvious distress he would definitely call out the vet, but that's about it. I wouldn't count on him being able to identify any distress short of a horse thrashing in a stall.
It's doable, but if you're going to be on the property then I would be willing to accept the fact that you must learn a little about horses and be willing to at a minimum call the emergency vet and owner out if necessary. As far as non-emergency interactions go, you can avoid them. I can't give any advice about the legal of side of things other than to get everything in writing and signed.