Melanoma in gray horse? Vet help?

I am thinking of buying a gray Arabian horse that the owners tell me have melanoma. I have talked to the vet that checked him and he said it was incredibly mild, they're just some tiny black bumps, and that its something I could wait a couple years before I even need to think about treating it. But I want to know first of all, is that true? That with a horse with mild melanoma you just wait for it to get worse before you treat it? And second of all, what options are out there for treatment? I know surgical removal has its pros and cons I've heard, so what other options are out there? And how much money can I expect to spend on treatment?