Hi, I’m really enjoying my 2018 SL400. I Think at the time the AMG line was the top spec but, IIRC, there were a few special editions as the r232 was waiting in the wings.
The air scarf and wind deflector are must haves, then over options are really a personal choice. I’ve read that the ABC body control is excellent, but as I usually keep my cars for as long as possible, shied away from possible expensive problems later.
Do the usual checks when test driving - make sure everything works, especially the roof. Rear tyres are a rare sise and can be quite expensive.
The only problems I’ve had with mine are a recall for the boot latch, a sticky parking brake and a problem with the boot separator - possibly due to a windscreen replacement the day before - the boot and separator stuck open with the roof down had to drive to my indie like this. Was an easy fix the separator had to be manually reset.
Good luck in your search.
Mark