The SL is most definitely a future classic but not all of them. The late no ties models (with the square vertical headlights like the SL 70 AMG in the pictures) have depreciated a lot in today’s market compared to other versions of the SL.
The current version is holding up values pretty good.
The old models with the vertical square headlights are a fortune.
Personally I got this one, a 1978 SL 500, holds value but still affordable (relatively speaking)