I feel for all of you in this situation, the owners because they have a cool horse who has a lot to offer a young rider, and for you because you have a young rider who could probably enjoy this horse. For a while.
A good friend bought a gray TB and at the vet check, the vet pointed out the melanomas and cautioned them strongly against him. She bought him, evented him, took him to the vet school to confirm that his head tossing/staring was related to the melanomas hear his head, later passed him along to me to ride (I LOVED that horse) and then use for kids’ lessons. His end came at age 22 from colic caused by all the melanomas in his rectum causing a horrible impaction - such an unfair end for a great horse. His melanomas were bulky and later very numerous, both on the outside and inside of his body.
Over the years I’ve had a gray QH and a gray Arab, both with melanomas, but neither with problems. The gray QH only had them under her tail, not very many, and never a problem. The gray Arab had one when I bought him, the size of a pea (excluded on his insurance) and at the time of his death at age 31, it was about the size of a golf ball. He had a few others, small round hard ones, but I had to look for them.
I have two more gray geldings now, one full Arab and one half, and so far at ages 8 and 12, no melanomas. So for my sample size of 5, I guess I have all ends of the spectrum, so far. Would I buy another gray horse? of course. 
Would I buy another gray horse with obvious, bulky melanomas, at any age? no.
It’s too bad the family wouldn’t consider leasing the horse to you, understanding that he’s pretty much unsellable, and letting your daughter benefit from all he knows. The tricky part might be figuring out how much intervention is appropriate for all the different tumors he appears to have, and who would pay for it.
