Pol, here is a thought one getting the “rights” for purchase of your grandfather’s planes. In real estate buyers will sometimes pay an owner a small fee to enter into an option. The option gives the holder the right to buy the property if he so chooses. Generally in real estate the option has a duration and purchase price. I’m sure that the format could be modified so that it is open ended and price to be negotiated at the time of sale. That way when the current owner comes to the point of selling you would get the “right of first refusal”. To make the contract binding you would likely have to pay a small fee to the owner. Good luck on your bid to reclaim some family history.
“Give me a woman who loves beer and I will conquer the world” - Kaiser Wilhelm
"Wasser ist für waschen, nicht trinkens. Bier ist für trinkens"
translation: Water is for washing not drinking, beer is for drinking. Poppa Boxhiemer