There are plenty of old grey who don't have Melanomas - in my case it is just once bitten twice shy. It was not a nice way to lose a horse and he was only 18.
I went to see an 8 yr old dapple grey mare with large golfball sized lump on neck with hard tissue around it - stunning mare with an amazing jump, but thought was suspicious of melanoma so walked away - may have been one of the best i saw but couldn't take the risk plus a biopsy and vetting would have been very costly.
I have had several grey horses without having any melanomas but did know of a chestnut arab that was peppered with them, he was relatively young when diagnosed and carried on to a great age.
I considered buying a 14 year old grey with the hugest Melanomas you have ever seen he looked like a hamster. Unfortunately the woman wanted the same money for him as you would expect to pay for a horse without melanomas of the same age and standard and wouldnt budge as in her opinion they didnt bother him. IMO however he still had stuff wrong with him and she should have been more open to offers.
So what I am trying to say is if the expectations and money is right then go for it.