What alternatives should I consider to a used Mercedes SL-Class sports?
The last of the Jaguar XKs could be seen as a sportier alternative to the SL thanks to its bombastic V8 and lightweight construction. You will have to offset that against the fact that the Jaguar is the far older design, so it won't have the same wow factor when it comes to technology or infotainment.
If you really did want a sports convertible then look no further than the Porsche 911. Unlike the SL, the 911 does actually come with two rear seats with proper belts, but don't expect anyone to be able to sit there without you scooting forward slightly. Best to think of those chairs as a place to supplement the small boot that can be found in the nose of the car. Aside from that, the 911 will leave an SL for dead on a challenging B road.