You need to determine exactly what gave way...
The actual Reciever/Dryer ?
The pressure switch on the R/D ?
The line going into it ?
Etc....

The green stuff was likely leak finder installed in the system.... and it sure worked for you .... you know you leaked refrigerant....

You will need to do a lot of reading in order to determine if you need to correct anything before you put it all back together.. because if you don't it is not likely to last long....

Stay with R12 if you can... you can read the "Air Conditioning Wars" by searching for" R12 alternatives".....in the search feature...

Mercedes published a separate manual with regards to the ' heating, airconditioning, and automatic climate control" You might consider acquiring one...