As someone actively leasing - prices are all over the place nowadays. The 1/3 the purchase price does not apply as widely as it did even 5 years ago.
To answer the OP’s question: a solid large that you can lease over and over again for SS will bring in a different price than a large that can do the children’s division. I don’t think I’d want to own the SS Large as an investment right out of the gate. But if it could do the children’s and then eventually need to step down and do the SS - sure? Question is - what do you do with the pony when she needs to retire?
And yes it could get hurt and need a retirement sooner rather than later, but you sound horse savvy enough to know that part…