I left France a while ago and things have changed since then. But here are a few indications:
You have to start with the "service des mines". They are the ones who will say that your car is fit for driving on French roads (in the past, the service des mines in the Alpes Marritimes was said to be quite accomodating, hope it's still true).
For that, they will need the "fiche des mines constructeur" or something named close to that. There should not be any problem to get it as the 230SL was imported in France, so I guess you could get it via a MB dealer, or the Mercedes Club de France.
They may also need the Controle Technique, unless it is the latter who will need a paper from the service des mines, I don't know in which order it is.
I don't think the 230SL will be tested for emissions, I think it's only for cars post 1970. Once you get all that you go to the Prefecture des Alpes marritimes to get your carte grise. I stronly recommend you to go for the carte grise "normale" as the "collection" one will have your car drop in value due to the restrictions in use it implies. Here things have changed and restrictions are less than before, but I don't know the details. Anyway, all my cars are with a carte grise "normale".
Yes, you can have your car insured at a better rate even with a carte grise "normale". the Assurance Collection are plenty around. You only need to have another car insured as your daily driver. This can be in another country, which is my case.
With all the MBs that sold in the area you will surely find a garage specialised in older MBs. Garage Simondi in Vance, if still in existence, will do all you need on your car. They may be others, but I'm not from there, so I don't know.