What do I say in a letter to someone who owns an abandoned house. That I would like to buy. Any ideas. I need your help

It sounds like you have plans for this house and you want to put life back in it. That would be a great start. Does this person have a past with this home ? If so than use that as an angle. No one wants to see a home they have had for years and have a past with, run down to the point it has to be torn down. Speak from your heart.