You seem decided on what you want ….. a V8 Calais. This may fit the bill.
https://www.carsales.com.au/private/details/Holden-Calais-1999/SSE-AD-5630739/?Cr=3
My reasoning, it’s the last of the Aussie V8 engines, which may be a collectable factor. Bulletproof, apart from the rear main seal. And you can upgrade the seating to leather from a later model.
But I have to question your idea of putting it away for years, for grandkid’s inheritance. Enjoy it yourself until you can’t, then put it away.
A car such as this in 30 years time may be an oddity, a museum piece, with future driver’s unable to use it because it has to be ‘driven’ without the aid of AEB, lane keeping, adaptive cruise control, and all the other electronic crap which seems to mandatory to keep people safe from themselves. Kids from my childhood rode horses and push bikes. Too dangerous now.
I got an email from Holden telling me how much ‘fun’ driving the new Acadia will be with all the electronic driver aids. My definition of ‘fun’ is a long way from that. ( must take the dust cover off the Monaro, and drive somewhere. )