What solid state unit are you looking for? The central locking system of the SDL runs off a pneumatic pump. Check the archives for fitting a remote keyless entry system to such a system. You need a system that can be configured to provide a pulse for something like 2 seconds. Many but not all RKE systems have such a feature.

Sixto
95 S420
87 300SDL
83 300SD ... $1100, see cars forum