One of my neighbours tried something very similar a few years ago. He was a farrier and had 9 acres. It was turned down.
I think the problem is that it is extemely difficult to make a living on such a small acreage.
From a business point of view this would be fairly costly to set up. How many horses would you need to take in, with full livery, in order to make a decent living?
Normally you might keep up to 5 horses on 5 acres. Would that actually pay enough? How many stables would you need and how much would that cost?
How much track are you planning to put down? That in itself involves digging out and putting down hardcore, with then the gravel on top. That's likely to cost more than the stables.
You asked for advice, so - not wanting to be horrible or brutal, and please don't take offence - this is my take on it:
Your daughter and boyfriend are presumably in their 20s? With their whole life ahead of them. She's likely to be pretty well paid as a vet, and I don't know what he does; but it's highly likely that once she's qualified and got established that they'll be able to buy their own place.
You and your husband, on the other hand, have little savings and no works pension, so I would have thought you (and especially your daughter and boyfriend) should be more looking to how you can best make your land and buildings work for YOU, so you have some security and income when you retire.