buy legacy, better still.
Good lens hard to find
Legendary lenses will appreciate in value
Just ask MWP about his CY Zeiss...
Unfortunately, I am not an astute lens investor. I sold most of my Leica R lenses before they shoot up in prices. But then I have never thought of buying them as investments. Also the Singapore buyers are too meow. They usually will not go in for a higher than normal priced lens. See how long that CY50/1.7 and Voigt125 have been on BnS. And add the lemons that you get from the used markets, I would be selective too.